Tuesday, December 15, 2015

Controlling the SABR wings with Hagan PDE

On the Wilmott forum, Pat Hagan has recently suggested to cap the equivalent local volatility in order to control the wings and better match CMS prices. It also helps making the SABR approximation better behaved as the expansion is only valid when
$$1 + 2\frac{\rho\nu}{\alpha}y(K)+\frac{\nu^2}{\alpha^2}y^2(K)$$
is close to 1.

In the PDE approach (especially the non transformed one), it is very simple, one just needs to update the equivalent local vol as
$$\alpha K^\beta \min\left(M, \sqrt{1 + 2\frac{\rho\nu}{\alpha}y(K)+\frac{\nu^2}{\alpha^2}y^2(K)}\right)$$

While it is straightforward to include in the PDE, it is more difficult to derive a good approximation. The zero-th order behaves as expected, but the first order formula has a unnatural kink, likely because of the non differentiability due to the min function.

The following graphs presents the non capped PDE, the capped PDE with M=4*nu (PDEC4) and M=6*nu (PDEC6) as well as the approximation (Andersen Ratcliffe / Gatheral first order) where I have only taken care of the right wing. The SABR parameters are alpha = 0.0630, beta = 0.7, rho = -0.363, nu = 0.421, T = 10, f = 0.0439.

We can see that the higher the cap is, the closer we are to the standard SABR PDE, and the lower the cap is, the flatter are the wings.

The approximation matches well ATM (it is then equivalent to standard SABR PDE) but then has a discontinuous derivative for the K that reaches the threshold M. Far away, it matches very well again.

Controlling the SABR wings with Hagan PDE

On the Wilmott forum, Pat Hagan has recently suggested to cap the equivalent local volatility in order to control the wings and better match CMS prices. It also helps making the SABR approximation better behaved as the expansion is only valid when
$$1 + 2\frac{\rho\nu}{\alpha}y(K)+\frac{\nu^2}{\alpha^2}y^2(K)$$
is close to 1.

In the PDE approach (especially the non transformed one), it is very simple, one just needs to update the equivalent local vol as
$$\alpha K^\beta \min\left(M, \sqrt{1 + 2\frac{\rho\nu}{\alpha}y(K)+\frac{\nu^2}{\alpha^2}y^2(K)}\right)$$

While it is straightforward to include in the PDE, it is more difficult to derive a good approximation. The zero-th order behaves as expected, but the first order formula has a unnatural kink, likely because of the non differentiability due to the min function.

The following graphs presents the non capped PDE, the capped PDE with M=4*nu (PDEC4) and M=6*nu (PDEC6) as well as the approximation (Andersen Ratcliffe / Gatheral first order) where I have only taken care of the right wing. The SABR parameters are alpha = 0.0630, beta = 0.7, rho = -0.363, nu = 0.421, T = 10, f = 0.0439.

We can see that the higher the cap is, the closer we are to the standard SABR PDE, and the lower the cap is, the flatter are the wings.

The approximation matches well ATM (it is then equivalent to standard SABR PDE) but then has a discontinuous derivative for the K that reaches the threshold M. Far away, it matches very well again.

Monday, November 09, 2015

Broken Internet?

There is something funny going on with upcoming generic top level domains (gTLDs), they seem to be looked up in a strange manner (at least on latest Linux). For example:

ping chrome

or

ping nexus

returns 127.0.53.53.

While existing official gTLDs don't (ping dental returns "unknown host" as expected). I first thought it was a network misconfiguration, but as I am not the only one to notice this, it's likely a genuine internet issue.

Strange times.

Broken Internet?

There is something funny going on with upcoming generic top level domains (gTLDs), they seem to be looked up in a strange manner (at least on latest Linux). For example:

ping chrome

or

ping nexus

returns 127.0.53.53.

While existing official gTLDs don't (ping dental returns "unknown host" as expected). I first thought it was a network misconfiguration, but as I am not the only one to notice this, it's likely a genuine internet issue.

Strange times.

Sunday, November 01, 2015

Holiday's read - DFW - Everything and more

I am ambivalent towards David Foster Wallace. He can write the most creative sentences and make innocuous subjects very interesting. At the same time, i never finished his book Infinite Jest, partly because the characters names are too awkward for me so that i never exactly remember who is who, but also because the story itself is a bit too crazy.
I knew however that a non fiction book on the subject of infinity written by him would make for a very interesting read. And I have not been disappointed. It's in between maths and philosophy going back to the Greeks up to Gödel through a lot of Cantor following more or less the historical chronology.
Most of it is easy to read and follow, except the last part around sets and transfinite numbers. This last part is actually quite significant as it tries to explain why we still have no satisfying theory around the problems raised by infinity especially in the context of a Sets theory. I did not expect to learn much around the subject, I was disappointed. The book showed me how naive I was and how tricky the concept of infinity can be.
While I found the different explanations around Zeno's paradox of the arrow very clever, there is one other view possible: the arrow really does not move at each instant (you could think of those as a snapshot) but an interval of time is just not a simple succession of instants. This is not so far of Aristotle attack, but the key here is around what is an interval really. DFW suggests slightly this interpretation as well p144 but it's not very explicit.
I had not heard about Kronecker's conception that only integers were mathematically real (against decimals, irrationals, infinite sets). I find it very appropriate in the frame of computer science. Everything ends up as finite integers (a binary representation) and we are always confronted to the process of transforming the continuous, that despite all its conceptual issues is often simpler to reason in to solve concrete problems, to the finite discrete.

Holiday's read - DFW - Everything and more

I am ambivalent towards David Foster Wallace. He can write the most creative sentences and make innocuous subjects very interesting. At the same time, i never finished his book Infinite Jest, partly because the characters names are too awkward for me so that i never exactly remember who is who, but also because the story itself is a bit too crazy.
I knew however that a non fiction book on the subject of infinity written by him would make for a very interesting read. And I have not been disappointed. It's in between maths and philosophy going back to the Greeks up to Gödel through a lot of Cantor following more or less the historical chronology.
Most of it is easy to read and follow, except the last part around sets and transfinite numbers. This last part is actually quite significant as it tries to explain why we still have no satisfying theory around the problems raised by infinity especially in the context of a Sets theory. I did not expect to learn much around the subject, I was disappointed. The book showed me how naive I was and how tricky the concept of infinity can be.
While I found the different explanations around Zeno's paradox of the arrow very clever, there is one other view possible: the arrow really does not move at each instant (you could think of those as a snapshot) but an interval of time is just not a simple succession of instants. This is not so far of Aristotle attack, but the key here is around what is an interval really. DFW suggests slightly this interpretation as well p144 but it's not very explicit.
I had not heard about Kronecker's conception that only integers were mathematically real (against decimals, irrationals, infinite sets). I find it very appropriate in the frame of computer science. Everything ends up as finite integers (a binary representation) and we are always confronted to the process of transforming the continuous, that despite all its conceptual issues is often simpler to reason in to solve concrete problems, to the finite discrete.

Wednesday, September 30, 2015

Crank-Nicolson and Rannacher Issues with Touch options

I just stumbled upon this particularly illustrative case where the Crank-Nicolson finite difference scheme behaves badly, and the Rannacher smoothing (2-steps backward Euler) is less than ideal: double one touch and double no touch options.

It is particularly evident when the option is sure to be hit, for example when the barriers are narrow, that is our delta should be around zero as well as our gamma. Let's consider a double one touch option with spot=100, upBarrier=101, downBarrier=99.9, vol=20%, T=1 month and a payout of 50K.
 Crank-Nicolson shows big spikes in the delta near the boundary
 Rannacher shows spikes in the delta as well
Crank-Nicolson spikes are so high that the price is actually a off itself.

The Rannacher smoothing reduces the spikes by 100x but it's still quite high, and would be higher had we placed the spot closer to the boundary. The gamma is worse. Note that we applied the smoothing only at maturity. In reality as the barrier is continuous, the smoothing should really be applied at each step, but then the scheme would be not so different from a simple Backward Euler.

In contrast, with a proper second order finite difference scheme, there is no spike.
 Delta with the TR-BDF2 finite difference method - the scale goes from -0.00008 to 0.00008.
 Delta with the Lawson-Morris finite difference scheme - the scale goes from -0.00005 to 0.00005
Both TR-BDF2 and Lawson-Morris (based on a local Richardson extrapolation of backward Euler) have a very low delta error, similarly, their gamma is very clean. This is reminiscent of the behavior on American options, but the effect is magnified here.

Crank-Nicolson and Rannacher Issues with Touch options

I just stumbled upon this particularly illustrative case where the Crank-Nicolson finite difference scheme behaves badly, and the Rannacher smoothing (2-steps backward Euler) is less than ideal: double one touch and double no touch options.

It is particularly evident when the option is sure to be hit, for example when the barriers are narrow, that is our delta should be around zero as well as our gamma. Let's consider a double one touch option with spot=100, upBarrier=101, downBarrier=99.9, vol=20%, T=1 month and a payout of 50K.
 Crank-Nicolson shows big spikes in the delta near the boundary
 Rannacher shows spikes in the delta as well
Crank-Nicolson spikes are so high that the price is actually a off itself.

The Rannacher smoothing reduces the spikes by 100x but it's still quite high, and would be higher had we placed the spot closer to the boundary. The gamma is worse. Note that we applied the smoothing only at maturity. In reality as the barrier is continuous, the smoothing should really be applied at each step, but then the scheme would be not so different from a simple Backward Euler.

In contrast, with a proper second order finite difference scheme, there is no spike.
 Delta with the TR-BDF2 finite difference method - the scale goes from -0.00008 to 0.00008.
 Delta with the Lawson-Morris finite difference scheme - the scale goes from -0.00005 to 0.00005
Both TR-BDF2 and Lawson-Morris (based on a local Richardson extrapolation of backward Euler) have a very low delta error, similarly, their gamma is very clean. This is reminiscent of the behavior on American options, but the effect is magnified here.

Wednesday, September 02, 2015

Clouds

I was wondering how to generate some nice cloudy like texture with a simple program. I first thought about using the Brownian motion, but of course if one uses it raw, with one pixel representing one movement in time, it's just going to look like a very noisy and grainy picture like this:
 Normal noise

There is however a nice continuous representation of the Brownian motion : the Paley-Wiener representation

This can produce an interesting smooth pattern, but it is just 1D. In the following picture, I apply it to each row (the column index being time), and then for each column (the row index being time). Of course this produces a symmetric picture, especially as I reused the same random numbers
If I use new random numbers for the columns, it is still symmetric, but destructive rather than constructive.

It turns out that spatial processes are something more complex than I first imagined. It is not a simple as using a N-dimensional Brownian motion, as it would produce a very similar picture as the 1-dimensional one. But this paper has a nice overview of spatial processes. Interestingly they even suggest to generate a Gaussian process using a Precision matrix (inverse of covariance matrix). I never thought about doing such a thing and I am not sure what is the advantage of such a scheme.

There is a standard graphic technique to generate nice textures, originating from Ken Perlin for Disney, it is called simply Perlin Noise. It turns out that several web pages in the top Google results confuse simple Perlin noise with fractal sum of noise that Ken Perlin also helped popularize (see his slides: standard Perlin noise, fractal noise). Those pages also believe that the later is simpler/faster. But there are two issues with fractal sum of noise: the first one is that it relies on an existing noise function - you need to first build one (it can be done with a random number generator and an interpolator), and the second one is that it ends up being more complex to program and likely to evaluate as well, see for example the code needed here. The fractal sum of noise is really a complementary technique.

The insight of Perlin noise is to not generate random color values that would be assigned to shades of grey as in my examples, but to generate random gradients, and interpolate on those gradient in a smooth manner. In computer graphics they like the cosine function to give a little bit of non-linearity in the colors. A good approximation, usually used as a replacement in this context is 3x^2 - 2x^3. It's not much more complicated than that, this web page explains it in great details. It can be programmed in a few lines of code.

 very procedural and non-optimized Go code for Perlin noise

Clouds

I was wondering how to generate some nice cloudy like texture with a simple program. I first thought about using the Brownian motion, but of course if one uses it raw, with one pixel representing one movement in time, it's just going to look like a very noisy and grainy picture like this:
 Normal noise

There is however a nice continuous representation of the Brownian motion : the Paley-Wiener representation

This can produce an interesting smooth pattern, but it is just 1D. In the following picture, I apply it to each row (the column index being time), and then for each column (the row index being time). Of course this produces a symmetric picture, especially as I reused the same random numbers
If I use new random numbers for the columns, it is still symmetric, but destructive rather than constructive.

It turns out that spatial processes are something more complex than I first imagined. It is not a simple as using a N-dimensional Brownian motion, as it would produce a very similar picture as the 1-dimensional one. But this paper has a nice overview of spatial processes. Interestingly they even suggest to generate a Gaussian process using a Precision matrix (inverse of covariance matrix). I never thought about doing such a thing and I am not sure what is the advantage of such a scheme.

There is a standard graphic technique to generate nice textures, originating from Ken Perlin for Disney, it is called simply Perlin Noise. It turns out that several web pages in the top Google results confuse simple Perlin noise with fractal sum of noise that Ken Perlin also helped popularize (see his slides: standard Perlin noise, fractal noise). Those pages also believe that the later is simpler/faster. But there are two issues with fractal sum of noise: the first one is that it relies on an existing noise function - you need to first build one (it can be done with a random number generator and an interpolator), and the second one is that it ends up being more complex to program and likely to evaluate as well, see for example the code needed here. The fractal sum of noise is really a complementary technique.

The insight of Perlin noise is to not generate random color values that would be assigned to shades of grey as in my examples, but to generate random gradients, and interpolate on those gradient in a smooth manner. In computer graphics they like the cosine function to give a little bit of non-linearity in the colors. A good approximation, usually used as a replacement in this context is 3x^2 - 2x^3. It's not much more complicated than that, this web page explains it in great details. It can be programmed in a few lines of code.

 very procedural and non-optimized Go code for Perlin noise

Saturday, August 22, 2015

Go for Monte-Carlo

I have looked a few months ago already at Julia, Dart, Rust and Scala programming languages to see how practical they could be for a simple Monte-Carlo option pricing.

I forgot the Go language. I had tried it 1 or 2 years ago, and at that time, did not enjoy it too much. Looking at Go 1.5 benchmarks on the computer language shootout, I was surprised that it seemed so close to Java performance now, while having a GC that guarantees pauses of less 10ms and consuming much less memory.

I am in general a bit skeptical about those benchmarks, some can be rigged. A few years ago, I tried my hand at the thread ring test, and found that it actually performed fastest on a single thread while it is supposed to measure the language threading performance. I looked yesterday at one Go source code (I think it was for pidigits) and saw that it just called a C library (gmp) to compute with big integers. It's no surprise then that Go would be faster than Java on this test.

So what about my small Monte-Carlo test?
Well it turns out that Go is quite fast on it:
Multipl. Rust    Go
1        0.005  0.007
10       0.03   0.03
100      0.21   0.29

1000     2.01   2.88

It is faster than Java/Scala and not too far off Rust, except if one uses FastMath in Scala, then the longest test is slighly faster with Java (not the other ones).

There are some issues with the Go language: there is no operator overloading, which can make matrix/vector algebra more tedious and there is no generic/template. The later is somewhat mitigated by the automatic interface implementation. And fortunately for the former, complex numbers are a standard type. Still, automatic differentiation would be painful.

Still it was extremely quick to grasp and write code, because it's so simple, especially when compared to Rust. But then, contrary to Rust, there is not as much safety provided by the language. Rust is quite impressive on this side (but unfortunately that implies less readable code). I'd say that Go could become a serious alternative to Java.

I also found an interesting minor performance issue with the default Go Rand.Float64, the library convert an Int63 to a double precision number this way:

func (r *Rand) Float64() float64 {
  f := float64(r.Int63()) / (1 << 63)  if f == 1 {    f = 0  }  return f }

I was interested in having a number in (0,1) and not [0,1), so I just used the conversion pattern from MersenneTwister 64 code:

f := (float64(r.Int63() >> 11) + 0.5) * (1.0/4503599627370496.0)

The reasoning behind this later code is that the mantissa is 52 bits, and this is the most accuracy we can have between 0 and 1. There is no need to go further, this also avoids the issue around 1. It's also straightforward that is will preserve the uniform property, while it's not so clear to me that r.Int63()/2^63 is going to preserve uniformity as double accuracy is higher around 0 (as the exponent part can be used there) and lesser around 1: there is going to be much more multiple identical results near 1 than near 0.

It turns out that the if check adds 5% performance penalty on this test, likely because of processor caching issues. I was surprised by that since there are many other ifs afterwards in the code, for the inverse cumulative function, and for the payoff.

Go for Monte-Carlo

I have looked a few months ago already at Julia, Dart, Rust and Scala programming languages to see how practical they could be for a simple Monte-Carlo option pricing.

I forgot the Go language. I had tried it 1 or 2 years ago, and at that time, did not enjoy it too much. Looking at Go 1.5 benchmarks on the computer language shootout, I was surprised that it seemed so close to Java performance now, while having a GC that guarantees pauses of less 10ms and consuming much less memory.

I am in general a bit skeptical about those benchmarks, some can be rigged. A few years ago, I tried my hand at the thread ring test, and found that it actually performed fastest on a single thread while it is supposed to measure the language threading performance. I looked yesterday at one Go source code (I think it was for pidigits) and saw that it just called a C library (gmp) to compute with big integers. It's no surprise then that Go would be faster than Java on this test.

So what about my small Monte-Carlo test?
Well it turns out that Go is quite fast on it:
Multipl. Rust    Go
1        0.005  0.007
10       0.03   0.03
100      0.21   0.29

1000     2.01   2.88

It is faster than Java/Scala and not too far off Rust, except if one uses FastMath in Scala, then the longest test is slighly faster with Java (not the other ones).

There are some issues with the Go language: there is no operator overloading, which can make matrix/vector algebra more tedious and there is no generic/template. The later is somewhat mitigated by the automatic interface implementation. And fortunately for the former, complex numbers are a standard type. Still, automatic differentiation would be painful.

Still it was extremely quick to grasp and write code, because it's so simple, especially when compared to Rust. But then, contrary to Rust, there is not as much safety provided by the language. Rust is quite impressive on this side (but unfortunately that implies less readable code). I'd say that Go could become a serious alternative to Java.

I also found an interesting minor performance issue with the default Go Rand.Float64, the library convert an Int63 to a double precision number this way:

func (r *Rand) Float64() float64 {
  f := float64(r.Int63()) / (1 << 63)
if f == 1 {
f = 0
}
return f
}

I was interested in having a number in (0,1) and not [0,1), so I just used the conversion pattern from MersenneTwister 64 code:

f := (float64(r.Int63() >> 11) + 0.5) * (1.0/4503599627370496.0)

The reasoning behind this later code is that the mantissa is 52 bits, and this is the most accuracy we can have between 0 and 1. There is no need to go further, this also avoids the issue around 1. It's also straightforward that is will preserve the uniform property, while it's not so clear to me that r.Int63()/2^63 is going to preserve uniformity as double accuracy is higher around 0 (as the exponent part can be used there) and lesser around 1: there is going to be much more multiple identical results near 1 than near 0.

It turns out that the if check adds 5% performance penalty on this test, likely because of processor caching issues. I was surprised by that since there are many other ifs afterwards in the code, for the inverse cumulative function, and for the payoff.

Saturday, July 25, 2015

Bumping Correlations

In his book "Monte Carlo Methods in Finance", P. Jäckel explains a simple way to clean up a correlation matrix. When a given correlation matrix is not positive semi-definite, the idea is to do a singular value decomposition (SVD), replace the negative eigenvalues by 0, and renormalize the corresponding eigenvector accordingly.

One of the cited applications is "stress testing and scenario analysis for market risk" or "comparative pricing in order to ascertain the extent of correlation exposure for multi-asset derivatives", saying that "In many of these cases we end up with a matrix that is no longer positive semi-definite".

It turns out that if one bumps an invalid correlation matrix (the input), that is then cleaned up automatically, the effect can be a very different bump. Depending on how familiar you are with SVD, this could be more or less obvious from the procedure,

As a simple illustration I take the matrix representing 3 assets A, B, C with rho_ab = -0.6, rho_ac = rho_bc = -0.5.

1.00000  -0.60000  -0.50000
-0.60000   1.00000  -0.50000
-0.50000  -0.50000   1.00000

For those rho_ac and rho_bc, the correlation matrix is not positive definite unless rho_ab in in the range (-0.5, 1). One way to verify this is to use the fact that positive definiteness is equivalent to a positive determinant. The determinant will be 1 - 2*0.25 - rho_ab^2 + 2*0.25*rho_ab.

After using P. Jaeckel procedure, we end up with:

1.00000  -0.56299  -0.46745
-0.56299   1.00000  -0.46745
-0.46745  -0.46745   1.00000

If we bump now rho_bc by 1% (absolute), we end up after cleanup with:

1.00000  -0.56637  -0.47045
-0.56637   1.00000  -0.46081
-0.47045  -0.46081   1.00000

It turns out that rho_bc has changed by only 0.66% and rho_ac by -0.30%, rho_ab by -0.34%. So our initial bump (0,0,1) has been translated to a bump (-0.34, -0.30, 0.66). In other words, it does not work to compute sensitivities.

One can optimize to obtain the nearest correlation matrix in some norm. Jaeckel proposes a hypersphere decomposition based optimization, using as initial guess the SVD solution. Higham proposed a specific algorithm just for that purpose. It turns out that on this example, they will converge to the same solution (if we use the same norm). I tried out of curiosity to see if that would lead to some improvement. The first matrix becomes

1.00000  -0.56435  -0.46672
-0.56435   1.00000  -0.46672
-0.46672  -0.46672   1.00000

And the bumped one becomes

1.00000  -0.56766  -0.46984
-0.56766   1.00000  -0.46002
-0.46984  -0.46002   1.00000

We find back the same issue, rho_bc has changed by only 0.67%, rho_ac by -0.31% and rho_ab by -0.33%. We also see that the SVD correlation or the real near correlation matrix are quite close, as noticed by P. Jaeckel.

Of course, one should apply the bump directly to the cleaned up matrix, in which case it will actually work as expected, unless our bump produces another non positive definite matrix, and then we would have correlation leaking a bit everywhere. It's not entirely clear what kind of meaning the risk figures would have then.

Bumping Correlations

In his book "Monte Carlo Methods in Finance", P. Jäckel explains a simple way to clean up a correlation matrix. When a given correlation matrix is not positive semi-definite, the idea is to do a singular value decomposition (SVD), replace the negative eigenvalues by 0, and renormalize the corresponding eigenvector accordingly.

One of the cited applications is "stress testing and scenario analysis for market risk" or "comparative pricing in order to ascertain the extent of correlation exposure for multi-asset derivatives", saying that "In many of these cases we end up with a matrix that is no longer positive semi-definite".

It turns out that if one bumps an invalid correlation matrix (the input), that is then cleaned up automatically, the effect can be a very different bump. Depending on how familiar you are with SVD, this could be more or less obvious from the procedure,

As a simple illustration I take the matrix representing 3 assets A, B, C with rho_ab = -0.6, rho_ac = rho_bc = -0.5.

1.00000  -0.60000  -0.50000
-0.60000   1.00000  -0.50000
-0.50000  -0.50000   1.00000

For those rho_ac and rho_bc, the correlation matrix is not positive definite unless rho_ab in in the range (-0.5, 1). One way to verify this is to use the fact that positive definiteness is equivalent to a positive determinant. The determinant will be 1 - 2*0.25 - rho_ab^2 + 2*0.25*rho_ab.

After using P. Jaeckel procedure, we end up with:

1.00000  -0.56299  -0.46745
-0.56299   1.00000  -0.46745
-0.46745  -0.46745   1.00000

If we bump now rho_bc by 1% (absolute), we end up after cleanup with:

1.00000  -0.56637  -0.47045
-0.56637   1.00000  -0.46081
-0.47045  -0.46081   1.00000

It turns out that rho_bc has changed by only 0.66% and rho_ac by -0.30%, rho_ab by -0.34%. So our initial bump (0,0,1) has been translated to a bump (-0.34, -0.30, 0.66). In other words, it does not work to compute sensitivities.

One can optimize to obtain the nearest correlation matrix in some norm. Jaeckel proposes a hypersphere decomposition based optimization, using as initial guess the SVD solution. Higham proposed a specific algorithm just for that purpose. It turns out that on this example, they will converge to the same solution (if we use the same norm). I tried out of curiosity to see if that would lead to some improvement. The first matrix becomes

1.00000  -0.56435  -0.46672
-0.56435   1.00000  -0.46672
-0.46672  -0.46672   1.00000

And the bumped one becomes

1.00000  -0.56766  -0.46984
-0.56766   1.00000  -0.46002
-0.46984  -0.46002   1.00000

We find back the same issue, rho_bc has changed by only 0.67%, rho_ac by -0.31% and rho_ab by -0.33%. We also see that the SVD correlation or the real near correlation matrix are quite close, as noticed by P. Jaeckel.

Of course, one should apply the bump directly to the cleaned up matrix, in which case it will actually work as expected, unless our bump produces another non positive definite matrix, and then we would have correlation leaking a bit everywhere. It's not entirely clear what kind of meaning the risk figures would have then.

Monday, July 13, 2015

Andreasen Huge extrapolation

There are not many arbitrage free extrapolation schemes. Benaim et al. extrapolation is one of the few that claims it. However, despite the paper's title, it is not truely arbitrage free. The density might be positive, but the forward is not preserved by the implied density. It can also lead to wings that don't obey Lee's moments condition.

On a Wilmott forum, P. Caspers proposed the following counter-example based on extrapolating SABR: $$\alpha=15\%, \beta=80\%, \nu=50\%, \rho=-48\%, f=3\%, T=20.0$$. He cut this smile at 2.5% and 6% and used the BDK extrapolation scheme with mu=nu=1.

A truly arbitrage free extrapolation can be obtained through Andreasen Huge volatility interpolation, making sure the grid is wide enough to allow extrapolation. Their method is basically a one step finite difference implicit Euler scheme applied to a local volatility parameterization that has as many parameters than prices. The method is presented with piecewise constant local volatility, but actually used with piecewise linear local volatility in their example.
 Smile

 Density with piecewise linear local volatility
There is still a tiny oscillation that makes the density negative, but one understands why typical extrapolations fail on the example: the change in density must be very steep.
Note that moving the left extrapolation point even closer to the forward might fix BDK negative density, but we are already very close, and we can really wonder if going closer is really a good idea since we would effectively use a somewhat arbitrary extrapolation in most of the interpolation zone.

It turns out that we can also use a cubic spline local volatility with linear extrapolation, and the density would look then:
 Density with cubic spline local volatility
Interestingly, the right side of the density is much better captured.
The wiggle persists, although it is smaller. This is likely due to the fact that I am using a cubic spline on top of the finite difference prices (in order to have a C2 density). Using a better C2 convexity preserving interpolation would likely remove this artefact.

Those figures also show why relying just on extrapolation to fix SABR is not necessarily a good idea: even a real arbitrage free extrapolation will make a not so meaningful density. The proper solution is to really use Hagan's arbitrage free SABR PDE, which would be as nearly fast in this case.

Andreasen Huge extrapolation

There are not many arbitrage free extrapolation schemes. Benaim et al. extrapolation is one of the few that claims it. However, despite the paper's title, it is not truely arbitrage free. The density might be positive, but the forward is not preserved by the implied density. It can also lead to wings that don't obey Lee's moments condition.

On a Wilmott forum, P. Caspers proposed the following counter-example based on extrapolating SABR: $$\alpha=15\%, \beta=80\%, \nu=50\%, \rho=-48\%, f=3\%, T=20.0$$. He cut this smile at 2.5% and 6% and used the BDK extrapolation scheme with mu=nu=1.

A truly arbitrage free extrapolation can be obtained through Andreasen Huge volatility interpolation, making sure the grid is wide enough to allow extrapolation. Their method is basically a one step finite difference implicit Euler scheme applied to a local volatility parameterization that has as many parameters than prices. The method is presented with piecewise constant local volatility, but actually used with piecewise linear local volatility in their example.
 Smile

 Density with piecewise linear local volatility
There is still a tiny oscillation that makes the density negative, but one understands why typical extrapolations fail on the example: the change in density must be very steep.
Note that moving the left extrapolation point even closer to the forward might fix BDK negative density, but we are already very close, and we can really wonder if going closer is really a good idea since we would effectively use a somewhat arbitrary extrapolation in most of the interpolation zone.

It turns out that we can also use a cubic spline local volatility with linear extrapolation, and the density would look then:
 Density with cubic spline local volatility
Interestingly, the right side of the density is much better captured.
The wiggle persists, although it is smaller. This is likely due to the fact that I am using a cubic spline on top of the finite difference prices (in order to have a C2 density). Using a better C2 convexity preserving interpolation would likely remove this artefact.

Those figures also show why relying just on extrapolation to fix SABR is not necessarily a good idea: even a real arbitrage free extrapolation will make a not so meaningful density. The proper solution is to really use Hagan's arbitrage free SABR PDE, which would be as nearly fast in this case.

Tuesday, July 07, 2015

Unintuitive behavior of the Black-Scholes formula - negative volatilities in displaced diffusion extrapolation

I am looking at various extrapolation schemes of the implied volatilities. An interesting one I stumbled upon is due to Kahale. Even if his paper is on interpolation, there is actually a small paragraph on using the same kind of function for extrapolation. His idea is to simply lookup the standard deviation $$\Sigma$$ and the forward $$f$$ corresponding to a given market volatility and slope:
$$c_{f,\Sigma} = f N(d_1) - k N(d_2)$$
with
$$d_1 = \frac{\log(f/k)+\Sigma^2/2}{\Sigma}$$
We have simply:
$$c'(k) = - N(d_2)$$

He also proves that we can always find those two parameters for any $$k_0 > 0, c_0 > 0, -1 < c_0' < 0$$

Then I had the silly idea of trying to match with a put  instead of a call for the left wing (as those are out-of-the-money, and therefore easier to invert numerically). It turns out that it works in most cases in practice and produces relatively nice looking extrapolations, but it does not always work. This is because contrary to the call, the put value is bounded with $$f$$.
$$p_{f,\Sigma} = k N(-d_2) - f N(-d_1)$$
Inverting $$p_0'$$ is going to lead to a specific $$d_2$$, and you are not guaranteed that you can push $$f$$ high and have $$p_{f,\Sigma}$$ large enough to match $$p_0$$. As example we can just take $$p_0 \geq k N(-d_2)$$ which will only be matched if $$f \leq 0$$.

This is slightly unintuitive as put-call parity would suggest some kind of equivalence. The problem here is that we would need to consider the function of $$k$$ instead of $$f$$ for it to work, so we can't really work with a put directly.

Here are the two different extrapolations on Kahale own example:
 Extrapolation of the left wing with calls (blue doted line)

 Extrapolation of the left wing with puts (blue doted line)
Displaced diffusion extrapolation is sometimes advocated. It is not the same as Kahale extrapolation: In Kahale, only the forward variable is varying in the Black-Scholes formula, and there is no real underlying stochastic process. In a displaced diffusion setting, we would adjust both strike and forward, keeping put-call parity at the formula level. But unfortunately, it suffers from the same kind of problem: it can not always be solved for slope and price. When it can however, it will give a more consistent extrapolation.

I find it interesting that some smiles can not be extrapolated by displaced diffusion in a C1 manner except if one allows negative volatilities in the formula (in which case we are not anymore in a pure displaced diffusion setting).
 Extrapolation of the left wing using negative displaced diffusion volatilities (blue dotted line)

Unintuitive behavior of the Black-Scholes formula - negative volatilities in displaced diffusion extrapolation

I am looking at various extrapolation schemes of the implied volatilities. An interesting one I stumbled upon is due to Kahale. Even if his paper is on interpolation, there is actually a small paragraph on using the same kind of function for extrapolation. His idea is to simply lookup the standard deviation $$\Sigma$$ and the forward $$f$$ corresponding to a given market volatility and slope:
$$c_{f,\Sigma} = f N(d_1) - k N(d_2)$$
with
$$d_1 = \frac{\log(f/k)+\Sigma^2/2}{\Sigma}$$
We have simply:
$$c'(k) = - N(d_2)$$

He also proves that we can always find those two parameters for any $$k_0 > 0, c_0 > 0, -1 < c_0' < 0$$

Then I had the silly idea of trying to match with a put  instead of a call for the left wing (as those are out-of-the-money, and therefore easier to invert numerically). It turns out that it works in most cases in practice and produces relatively nice looking extrapolations, but it does not always work. This is because contrary to the call, the put value is bounded with $$f$$.
$$p_{f,\Sigma} = k N(-d_2) - f N(-d_1)$$
Inverting $$p_0'$$ is going to lead to a specific $$d_2$$, and you are not guaranteed that you can push $$f$$ high and have $$p_{f,\Sigma}$$ large enough to match $$p_0$$. As example we can just take $$p_0 \geq k N(-d_2)$$ which will only be matched if $$f \leq 0$$.

This is slightly unintuitive as put-call parity would suggest some kind of equivalence. The problem here is that we would need to consider the function of $$k$$ instead of $$f$$ for it to work, so we can't really work with a put directly.

Here are the two different extrapolations on Kahale own example:
 Extrapolation of the left wing with calls (blue doted line)

 Extrapolation of the left wing with puts (blue doted line)
Displaced diffusion extrapolation is sometimes advocated. It is not the same as Kahale extrapolation: In Kahale, only the forward variable is varying in the Black-Scholes formula, and there is no real underlying stochastic process. In a displaced diffusion setting, we would adjust both strike and forward, keeping put-call parity at the formula level. But unfortunately, it suffers from the same kind of problem: it can not always be solved for slope and price. When it can however, it will give a more consistent extrapolation.

I find it interesting that some smiles can not be extrapolated by displaced diffusion in a C1 manner except if one allows negative volatilities in the formula (in which case we are not anymore in a pure displaced diffusion setting).
 Extrapolation of the left wing using negative displaced diffusion volatilities (blue dotted line)

Wednesday, June 24, 2015

Linux Desktops in 2015

I seem to never be entirely happy with any of the linux desktops these days. I have used XFCE on Ubuntu quite a bit in the past year, it mostly works, but I still had minor annoyances:
- sometimes (rarely) my laptop would not wake up from sleep.
- notifications sometimes keep popping up too much.
- on my desktop, experienced strong tearing issues with the Radeon graphic card, except with some very specific combination of video player settings and desktop settings (and then I had annoying redraw issue when pushing volume up/down in movies).

I am satisfied with two different approaches since:
- OpenSuse 13.2 with KDE 4. I use that on my desktop, all issues are gone, and the integration of KDE in OpenSuse is clearly the best I have experienced. In contrast, KDE 5 on Ubuntu was a disaster for me. I also managed to fuck up the apt dependencies so much that I thought it would be simpler to reinstall a new distribution.
- Mate on Ubuntu 15.04. Very impressed so far. It's probably what Gnome should have been instead of going to 3.0. Even if there are nice aspects of the Gnome shell, Mate is fast, pretty, user friendly, much better than Cinnamon. There are even a few layouts to choose (most of them are good), here is "Eleven with Mate menu" (it installed and setup the Plank dock automatically for that layout, more traditional layouts without dock are available):

Linux Desktops in 2015

I seem to never be entirely happy with any of the linux desktops these days. I have used XFCE on Ubuntu quite a bit in the past year, it mostly works, but I still had minor annoyances:
- sometimes (rarely) my laptop would not wake up from sleep.
- notifications sometimes keep popping up too much.
- on my desktop, experienced strong tearing issues with the Radeon graphic card, except with some very specific combination of video player settings and desktop settings (and then I had annoying redraw issue when pushing volume up/down in movies).

I am satisfied with two different approaches since:
- OpenSuse 13.2 with KDE 4. I use that on my desktop, all issues are gone, and the integration of KDE in OpenSuse is clearly the best I have experienced. In contrast, KDE 5 on Ubuntu was a disaster for me. I also managed to fuck up the apt dependencies so much that I thought it would be simpler to reinstall a new distribution.
- Mate on Ubuntu 15.04. Very impressed so far. It's probably what Gnome should have been instead of going to 3.0. Even if there are nice aspects of the Gnome shell, Mate is fast, pretty, user friendly, much better than Cinnamon. There are even a few layouts to choose (most of them are good), here is "Eleven with Mate menu" (it installed and setup the Plank dock automatically for that layout, more traditional layouts without dock are available):

Friday, June 19, 2015

Square Root Crank-Nicolson

C. Reisinger kindly pointed out to me this paper around square root Crank-Nicolson. The idea is to apply a square root of time transformation to the PDE, and discretize the resulting PDE with Crank-Nicolson. Two reasons come to mind to try this:
• the square root transform will result in small steps initially, where the solution is potentially not so smooth, making Crank-Nicolson behave better.
•  it is the natural time of the Brownian motion.
Interestingly, it has nicer properties than what those reasons may suggest. On the Fokker-Planck density PDE, it does not oscillate under some very mild conditions and preserves density positivity at the peak.

Out of curiosity I tried it to price a one touch barrier option. Of course there is an analytical solution in my test case (Black-Scholes assumptions), but as soon as rates are assumed not constant or local volatility is used, there is no other solution than a numerical method. In the later case, finite difference methods are quite good in terms of performance vs accuracy.

The classic Crank-Nicolson gives a reasonable price, but the strong oscillations near the barrier, at every time step are not very comforting.
 Crank-Nicolson Prices near the Barrier. Each line is a different time.

Moving to square root of time removes nearly all oscillations on this problem, even with a relatively low number of time steps compared to the number of space steps.
 Square Root Crank-Nicolson Prices near the Barrier. Each line is a different time.

We can see that the second step prices are a bit higher than the third step (the lines cross), which looks like a small numerical oscillation in time, even if there is no oscillation is space.

 TR-BDF2 Prices near the Barrier. Each line is a different time.

As a comparison, the TR-BDF2 scheme does relatively well: oscillations are removed after the second step, even with the extreme ratio of time steps vs space steps used on this example so that illustrations are clearer - Crank-Nicolson would still oscillate a lot with 10 times less space steps but we would not see oscillation on the square root Crank-Nicolson and a very mild one on TR-BDF2.

The LMG2 scheme (a local richardson extrapolation) does not oscillate at all on this problem but is the slowest:
 LMG2 Prices near the Barrier. Each line is a different time.

The square root Crank-Nicolson is quite elegant. It can however not be applied to that many problems in practice, as often some grid times are imposed by the payoff to evaluate, for example in a case of a discrete weekly barrier. But for continuous time problems (density PDE, Vanilla, American, continuous barriers) it's quite good.

In reality, with a continuous barrier, the payoff is not discontinuous at every step, but it is only discontinuous at the first step. So Rannacher smoothing would work very well on that problem:
 Rannacher Prices near the Barrier. Each line is a different time.
The somewhat interesting payoff left for the square root Crank-Nicolson is the American.

Square Root Crank-Nicolson

C. Reisinger kindly pointed out to me this paper around square root Crank-Nicolson. The idea is to apply a square root of time transformation to the PDE, and discretize the resulting PDE with Crank-Nicolson. Two reasons come to mind to try this:
• the square root transform will result in small steps initially, where the solution is potentially not so smooth, making Crank-Nicolson behave better.
•  it is the natural time of the Brownian motion.
Interestingly, it has nicer properties than what those reasons may suggest. On the Fokker-Planck density PDE, it does not oscillate under some very mild conditions and preserves density positivity at the peak.

Out of curiosity I tried it to price a one touch barrier option. Of course there is an analytical solution in my test case (Black-Scholes assumptions), but as soon as rates are assumed not constant or local volatility is used, there is no other solution than a numerical method. In the later case, finite difference methods are quite good in terms of performance vs accuracy.

The classic Crank-Nicolson gives a reasonable price, but the strong oscillations near the barrier, at every time step are not very comforting.
 Crank-Nicolson Prices near the Barrier. Each line is a different time.

Moving to square root of time removes nearly all oscillations on this problem, even with a relatively low number of time steps compared to the number of space steps.
 Square Root Crank-Nicolson Prices near the Barrier. Each line is a different time.

We can see that the second step prices are a bit higher than the third step (the lines cross), which looks like a small numerical oscillation in time, even if there is no oscillation is space.

 TR-BDF2 Prices near the Barrier. Each line is a different time.

As a comparison, the TR-BDF2 scheme does relatively well: oscillations are removed after the second step, even with the extreme ratio of time steps vs space steps used on this example so that illustrations are clearer - Crank-Nicolson would still oscillate a lot with 10 times less space steps but we would not see oscillation on the square root Crank-Nicolson and a very mild one on TR-BDF2.

The LMG2 scheme (a local richardson extrapolation) does not oscillate at all on this problem but is the slowest:
 LMG2 Prices near the Barrier. Each line is a different time.

The square root Crank-Nicolson is quite elegant. It can however not be applied to that many problems in practice, as often some grid times are imposed by the payoff to evaluate, for example in a case of a discrete weekly barrier. But for continuous time problems (density PDE, Vanilla, American, continuous barriers) it's quite good.

In reality, with a continuous barrier, the payoff is not discontinuous at every step, but it is only discontinuous at the first step. So Rannacher smoothing would work very well on that problem:
 Rannacher Prices near the Barrier. Each line is a different time.
The somewhat interesting payoff left for the square root Crank-Nicolson is the American.