# simByEuler

Simulate Merton jump diffusion sample paths by Euler approximation

## Syntax

``[Paths,Times,Z,N] = simByEuler(MDL,NPeriods)``
``[Paths,Times,Z,N] = simByEuler(___,Name,Value)``

## Description

example

````[Paths,Times,Z,N] = simByEuler(MDL,NPeriods)` simulates `NTrials` sample paths of `NVars` correlated state variables driven by `NBrowns` Brownian motion sources of risk and `NJumps` compound Poisson processes representing the arrivals of important events over `NPeriods` consecutive observation periods. The simulation approximates the continuous-time Merton jump diffusion process by the Euler approach.```

example

````[Paths,Times,Z,N] = simByEuler(___,Name,Value)` specifies options using one or more name-value pair arguments in addition to the input arguments in the previous syntax.```

## Examples

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Create a `merton` object.

```AssetPrice = 80; Return = 0.03; Sigma = 0.16; JumpMean = 0.02; JumpVol = 0.08; JumpFreq = 2; mertonObj = merton(Return,Sigma,JumpFreq,JumpMean,JumpVol,... 'startstat',AssetPrice)```
```mertonObj = Class MERTON: Merton Jump Diffusion ---------------------------------------- Dimensions: State = 1, Brownian = 1 ---------------------------------------- StartTime: 0 StartState: 80 Correlation: 1 Drift: drift rate function F(t,X(t)) Diffusion: diffusion rate function G(t,X(t)) Simulation: simulation method/function simByEuler Sigma: 0.16 Return: 0.03 JumpFreq: 2 JumpMean: 0.02 JumpVol: 0.08 ```

Use `simByEuler` to simulate `NTrials` sample paths of `NVars` correlated state variables driven by `NBrowns` Brownian motion sources of risk and `NJumps` compound Poisson processes representing the arrivals of important events over `NPeriods` consecutive observation periods. The function `simByEuler` approximates a continuous-time Merton jump diffusion process by the Euler approach.

```NPeriods = 2; [Paths,Times,Z,N] = simByEuler(mertonObj,NPeriods)```
```Paths = 3×1 80.0000 266.5590 306.2600 ```
```Times = 3×1 0 1 2 ```
```Z = 2×1 1.8339 -2.2588 ```
```N = 2×1 1 2 ```

`Paths` is a `3`-by-`1` matrix. The only column is the path of the simulated `AssetPrice`. The output `Z` is a series of matrices used to generate the Brownian motion vector. The output `N` is a series of matrices used to generate jump vectors.

## Input Arguments

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Stochastic differential equation model, specified as a `merton` object. You can create a `merton` object using `merton`.

Data Types: `object`

Number of simulation periods, specified as a positive scalar integer. The value of `NPeriods` determines the number of rows of the simulated output series.

Data Types: `double`

### Name-Value Pair Arguments

Specify optional comma-separated pairs of `Name,Value` arguments. `Name` is the argument name and `Value` is the corresponding value. `Name` must appear inside quotes. You can specify several name and value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

Example: ```[Paths,Times,Z,N] = simByEuler(merton,NPeriods,'DeltaTimes',dt)```

Simulated trials (sample paths) of `NPeriods` observations each, specified as the comma-separated pair consisting of `'NTrials'` and a positive scalar integer.

Data Types: `double`

Positive time increments between observations, specified as the comma-separated pair consisting of `'DeltaTimes'` and a scalar or a `NPeriods`-by-`1` column vector.

`DeltaTimes` represents the familiar dt found in stochastic differential equations, and determines the times at which the simulated paths of the output state variables are reported.

Data Types: `double`

Number of intermediate time steps within each time increment dt (specified as `DeltaTimes`), specified as the comma-separated pair consisting of `'NSteps'` and a positive scalar integer.

The `simByEuler` function partitions each time increment dt into `NSteps` subintervals of length dt/`NSteps`, and refines the simulation by evaluating the simulated state vector at `NSteps − 1` intermediate points. Although `simByEuler` does not report the output state vector at these intermediate points, the refinement improves accuracy by allowing the simulation to more closely approximate the underlying continuous-time process.

Data Types: `double`

Flag to use antithetic sampling to generate the Gaussian random variates that drive the Brownian motion vector (Wiener processes), specified as the comma-separated pair consisting of `'Antithetic'` and a scalar numeric or logical `1` (`true`) or `0` (`false`).

When you specify `true`, `simByEuler` performs sampling such that all primary and antithetic paths are simulated and stored in successive matching pairs:

• Odd trials `(1,3,5,...)` correspond to the primary Gaussian paths.

• Even trials `(2,4,6,...)` are the matching antithetic paths of each pair derived by negating the Gaussian draws of the corresponding primary (odd) trial.

Note

If you specify an input noise process (see `Z`), `simByEuler` ignores the value of `Antithetic`.

Data Types: `logical`

Direct specification of the dependent random noise process for generating the Brownian motion vector (Wiener process) that drives the simulation, specified as the comma-separated pair consisting of `'Z'` and a function or as an ```(NPeriods ⨉ NSteps)```-by-`NBrowns`-by-`NTrials` three-dimensional array of dependent random variates.

Note

If you specify `Z` as a function, it must return an `NBrowns`-by-`1` column vector, and you must call it with two inputs:

• A real-valued scalar observation time t

• An `NVars`-by-`1` state vector Xt

Data Types: `double` | `function`

Dependent random counting process for generating the number of jumps, specified as the comma-separated pair consisting of `'N'` and a function or an (`NPeriods``NSteps`) -by-`NJumps`-by-`NTrials` three-dimensional array of dependent random variates.

If you specify a function, `N` must return an `NJumps`-by-`1` column vector, and you must call it with two inputs: a real-valued scalar observation time t followed by an `NVars`-by-`1` state vector Xt.

Data Types: `double` | `function`

Flag that indicates how the output array `Paths` is stored and returned, specified as the comma-separated pair consisting of `'StorePaths'` and a scalar numeric or logical `1` (`true`) or `0` (`false`).

If `StorePaths` is `true` (the default value) or is unspecified, `simByEuler` returns `Paths` as a three-dimensional time series array.

If `StorePaths` is `false` (logical `0`), `simByEuler` returns `Paths` as an empty matrix.

Data Types: `logical`

Sequence of end-of-period processes or state vector adjustments, specified as the comma-separated pair consisting of `'Processes'` and a function or cell array of functions of the form

`${X}_{t}=P\left(t,{X}_{t}\right)$`

The `simByEuler` function runs processing functions at each interpolation time. The functions must accept the current interpolation time t, and the current state vector Xt and return a state vector that can be an adjustment to the input state.

If you specify more than one processing function, `simByEuler` invokes the functions in the order in which they appear in the cell array. You can use this argument to specify boundary conditions, prevent negative prices, accumulate statistics, plot graphs, and more.

The end-of-period `Processes` argument allows you to terminate a given trial early. At the end of each time step, `simByEuler` tests the state vector Xt for an all-`NaN` condition. Thus, to signal an early termination of a given trial, all elements of the state vector Xt must be `NaN`. This test enables you to define a `Processes` function to signal early termination of a trial, and offers significant performance benefits in some situations (for example, pricing down-and-out barrier options).

Data Types: `cell` | `function`

## Output Arguments

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Simulated paths of correlated state variables, returned as an ```(NPeriods + 1)```-by-`NVars`-by-`NTrials` three-dimensional time series array.

For a given trial, each row of `Paths` is the transpose of the state vector Xt at time t. When `StorePaths` is set to `false`, `simByEuler` returns `Paths` as an empty matrix.

Observation times associated with the simulated paths, returned as an `(NPeriods + 1)`-by-`1` column vector. Each element of `Times` is associated with the corresponding row of `Paths`.

Dependent random variates used to generate the Brownian motion vector (Wiener processes) that drive the simulation, returned as an ```(NPeriods ⨉ NSteps)```-by-`NBrowns`-by-`NTrials` three-dimensional time-series array.

Dependent random variates for generating the jump counting process vector, returned as an ```(NPeriods ⨉ NSteps)```-by-`NJumps`-by-`NTrials` three-dimensional time-series array.

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### Antithetic Sampling

Simulation methods allow you to specify a popular variance reduction technique called antithetic sampling.

This technique attempts to replace one sequence of random observations with another that has the same expected value but a smaller variance. In a typical Monte Carlo simulation, each sample path is independent and represents an independent trial. However, antithetic sampling generates sample paths in pairs. The first path of the pair is referred to as the primary path, and the second as the antithetic path. Any given pair is independent other pairs, but the two paths within each pair are highly correlated. Antithetic sampling literature often recommends averaging the discounted payoffs of each pair, effectively halving the number of Monte Carlo trials.

This technique attempts to reduce variance by inducing negative dependence between paired input samples, ideally resulting in negative dependence between paired output samples. The greater the extent of negative dependence, the more effective antithetic sampling is.

## Algorithms

This function simulates any vector-valued SDE of the following form:

`$d{X}_{t}=B\left(t,{X}_{t}\right){X}_{t}dt+D\left(t,{X}_{t}\right)V\left(t,{x}_{t}\right)d{W}_{t}+Y\left(t,{X}_{t},{N}_{t}\right){X}_{t}d{N}_{t}$`

Here:

• Xt is an `NVars`-by-`1` state vector of process variables.

• B(t,Xt) is an `NVars`-by-`NVars` matrix of generalized expected instantaneous rates of return.

• `D(t,Xt)` is an `NVars`-by-`NVars` diagonal matrix in which each element along the main diagonal is the corresponding element of the state vector.

• `V(t,Xt)` is an `NVars`-by-`NVars` matrix of instantaneous volatility rates.

• dWt is an `NBrowns`-by-`1` Brownian motion vector.

• `Y(t,Xt,Nt)` is an `NVars`-by-`NJumps` matrix-valued jump size function.

• dNt is an `NJumps`-by-`1` counting process vector.

`simByEuler` simulates `NTrials` sample paths of `NVars` correlated state variables driven by `NBrowns` Brownian motion sources of risk over `NPeriods` consecutive observation periods, using the Euler approach to approximate continuous-time stochastic processes.

This simulation engine provides a discrete-time approximation of the underlying generalized continuous-time process. The simulation is derived directly from the stochastic differential equation of motion. Thus, the discrete-time process approaches the true continuous-time process only as `DeltaTimes` approaches zero.

## References

[1] Deelstra, Griselda, and Freddy Delbaen. “Convergence of Discretized Stochastic (Interest Rate) Processes with Stochastic Drift Term.” Applied Stochastic Models and Data Analysis. 14, no. 1, 1998, pp. 77–84.

[2] Higham, Desmond, and Xuerong Mao. “Convergence of Monte Carlo Simulations Involving the Mean-Reverting Square Root Process.” The Journal of Computational Finance 8, no. 3, (2005): 35–61.

[3] Lord, Roger, Remmert Koekkoek, and Dick Van Dijk. “A Comparison of Biased Simulation Schemes for Stochastic Volatility Models.” Quantitative Finance 10, no. 2 (February 2010): 177–94.

Introduced in R2020a