cir

Cox-Ingersoll-Ross mean-reverting square root diffusion model

Description

Creates and displays cir objects, which derive from the sdemrd (SDE with drift rate expressed in mean-reverting form) class.

Use cir objects to simulate sample paths of NVARS state variables expressed in mean-reverting drift-rate form. These state variables are driven by NBROWNS Brownian motion sources of risk over NPERIODS consecutive observation periods, approximating continuous-time CIR stochastic processes with square root diffusions.

You can simulate any vector-valued CIR process of the form:

$$d{X}_t=S(t)[L(t)-X_t]dt+D(t,X_t^{\frac{1}{2}})V(t)d{W}_t$$

where:

• X_t is an NVARS-by-1 state vector of process variables.

• S is an NVARS-by-NVARS matrix of mean reversion speeds (the rate of mean reversion).

• L is an NVARS-by-1 vector of mean reversion levels (long-run mean or level).

• D is an NVARS-by-NVARS diagonal matrix, where each element along the main diagonal is the square root of the corresponding element of the state vector.

• V is an NVARS-by-NBROWNS instantaneous volatility rate matrix.

• dW_t is an NBROWNS-by-1 Brownian motion vector.

Creation

Syntax

CIR = cir(Speed,Level,Sigma)

CIR = cir(___,Name,Value)

Description

CIR = cir(Speed,Level,Sigma) creates a default CIR object.

Specify required input parameters as one of the following types:

• A MATLAB^® array. Specifying an array indicates a static (non-time-varying) parametric specification. This array fully captures all implementation details, which are clearly associated with a parametric form.

• A MATLAB function. Specifying a function provides indirect support for virtually any static, dynamic, linear, or nonlinear model. This parameter is supported via an interface, because all implementation details are hidden and fully encapsulated by the function.

Note

You can specify combinations of array and function input parameters as needed.

Moreover, a parameter is identified as a deterministic function of time if the function accepts a scalar time t as its only input argument. Otherwise, a parameter is assumed to be a function of time t and state X(t) and is invoked with both input arguments.

CIR = cir(___,Name,Value) creates a CIR object with additional options specified by one or more Name,Value pair arguments.

Name is a property name and Value is its corresponding value. Name must appear inside single quotes (''). You can specify several name-value pair arguments in any order as Name1,Value1,…,NameN,ValueN

The CIR object has the following Properties:

• StartTime — Initial observation time

• StartState — Initial state at time StartTime

• Correlation — Access function for the Correlation input argument, callable as a function of time

• Drift — Composite drift-rate function, callable as a function of time and state

• Diffusion — Composite diffusion-rate function, callable as a function of time and state

• Simulation — A simulation function or method

• Speed — Access function for the input argument Speed, callable as a function of time and state

• Level — Access function for the input argument Level, callable as a function of time and state

• Sigma — Access function for the input argument Sigma, callable as a function of time and state

Input Arguments

Speed — Speed represents the parameter S

array or deterministic function of time or deterministic function of time and state

Speed represents the parameter S, specified as an array or deterministic function of time.

If you specify Speed as an array, it must be an NVARS-by-NVARS matrix of mean-reversion speeds (the rate at which the state vector reverts to its long-run average Level).

As a deterministic function of time, when Speed is called with a real-valued scalar time t as its only input, Speed must produce an NVARS-by-NVARS matrix. If you specify Speed as a function of time and state, it calculates the speed of mean reversion. This function must generate an NVARS-by-NVARS matrix of reversion rates when called with two inputs:

• A real-valued scalar observation time t.

• An NVARS-by-1 state vector X_t.

Data Types: double | function_handle

Level — Level represents the parameter L

array or deterministic function of time or deterministic function of time and state

Level represents the parameter L, specified as an array or deterministic function of time.

If you specify Level as an array, it must be an NVARS-by-1 column vector of reversion levels.

As a deterministic function of time, when Level is called with a real-valued scalar time t as its only input, Level must produce an NVARS-by-1 column vector. If you specify Level as a function of time and state, it must generate an NVARS-by-1 column vector of reversion levels when called with two inputs:

• A real-valued scalar observation time t.

• An NVARS-by-1 state vector X_t.

Data Types: double | function_handle

Sigma — Sigma represents the parameter V

array or deterministic function of time or deterministic function of time and state

Sigma represents the parameter V, specified as an array or a deterministic function of time.

If you specify Sigma as an array, it must be an NVARS-by-NBROWNS matrix of instantaneous volatility rates or as a deterministic function of time. In this case, each row of Sigma corresponds to a particular state variable. Each column corresponds to a particular Brownian source of uncertainty, and associates the magnitude of the exposure of state variables with sources of uncertainty.

As a deterministic function of time, when Sigma is called with a real-valued scalar time t as its only input, Sigma must produce an NVARS-by-NBROWNS matrix. If you specify Sigma as a function of time and state, it must return an NVARS-by-NBROWNS matrix of volatility rates when invoked with two inputs:

• A real-valued scalar observation time t.

• An NVARS-by-1 state vector X_t.

Data Types: double | function_handle

Note

Although cir does not enforce restrictions on the signs of these input arguments, each argument is specified as a positive value.

Properties

StartTime — Starting time of first observation, applied to all state variables

0 (default) | scalar

Starting time of first observation, applied to all state variables, specified as a scalar

Data Types: double

StartState — Initial values of state variables

1 (default) | scalar, column vector, or matrix

Initial values of state variables, specified as a scalar, column vector, or matrix.

If StartState is a scalar, cir applies the same initial value to all state variables on all trials.

If StartState is a column vector, cir applies a unique initial value to each state variable on all trials.

If StartState is a matrix, cir applies a unique initial value to each state variable on each trial.

Data Types: double

Correlation — Correlation between Gaussian random variates drawn to generate the Brownian motion vector (Wiener processes)

NBROWNS-by-NBROWNS identity matrix representing independent Gaussian processes (default) | positive semidefinite matrix | deterministic function

Correlation between Gaussian random variates drawn to generate the Brownian motion vector (Wiener processes), specified as an NBROWNS-by-NBROWNS positive semidefinite matrix, or as a deterministic function C(t) that accepts the current time t and returns an NBROWNS-by-NBROWNS positive semidefinite correlation matrix.

A Correlation matrix represents a static condition.

As a deterministic function of time, Correlation allows you to specify a dynamic correlation structure.

Data Types: double

Simulation — User-defined simulation function or SDE simulation method

simulation by Euler approximation (simByEuler) (default) | function | SDE simulation method

User-defined simulation function or SDE simulation method, specified as a function or SDE simulation method.

Data Types: function_handle

Drift — Drift rate component of continuous-time stochastic differential equations (SDEs)

value stored from drift-rate function (default) | drift object or function accessible by (t, X_t)

This property is read-only.

Drift rate component of continuous-time stochastic differential equations (SDEs), specified as a drift object or function accessible by (t, X_t.

The drift rate specification supports the simulation of sample paths of NVARS state variables driven by NBROWNS Brownian motion sources of risk over NPERIODS consecutive observation periods, approximating continuous-time stochastic processes.

The drift class allows you to create drift-rate objects using drift of the form:

$$F(t,X_t)=A(t)+B(t)X_t$$

where:

• A is an NVARS-by-1 vector-valued function accessible using the (t, X_t) interface.

• B is an NVARS-by-NVARS matrix-valued function accessible using the (t, X_t) interface.

The displayed parameters for a drift object are:

• Rate: The drift-rate function, F(t,X_t)

• A: The intercept term, A(t,X_t), of F(t,X_t)

• B: The first order term, B(t,X_t), of F(t,X_t)

A and B enable you to query the original inputs. The function stored in Rate fully encapsulates the combined effect of A and B.

When specified as MATLAB double arrays, the inputs A and B are clearly associated with a linear drift rate parametric form. However, specifying either A or B as a function allows you to customize virtually any drift rate specification.

Note

You can express drift and diffusion classes in the most general form to emphasize the functional (t, X_t) interface. However, you can specify the components A and B as functions that adhere to the common (t, X_t) interface, or as MATLAB arrays of appropriate dimension.

Example: F = drift(0, 0.1) % Drift rate function F(t,X)

Data Types: struct | double

Diffusion — Diffusion rate component of continuous-time stochastic differential equations (SDEs)

value stored from diffusion-rate function (default) | diffusion object or functions accessible by (t, X_t)

This property is read-only.

Diffusion rate component of continuous-time stochastic differential equations (SDEs), specified as a drift object or function accessible by (t, X_t.

The diffusion rate specification supports the simulation of sample paths of NVARS state variables driven by NBROWNS Brownian motion sources of risk over NPERIODS consecutive observation periods, approximating continuous-time stochastic processes.

The diffusion class allows you to create diffusion-rate objects using diffusion:

$$G(t,X_t)=D(t,X_t^{\alpha (t)})V(t)$$

where:

• D is an NVARS-by-NVARS diagonal matrix-valued function.

• Each diagonal element of D is the corresponding element of the state vector raised to the corresponding element of an exponent Alpha, which is an NVARS-by-1 vector-valued function.

• V is an NVARS-by-NBROWNS matrix-valued volatility rate function Sigma.

• Alpha and Sigma are also accessible using the (t, X_t) interface.

The displayed parameters for a diffusion object are:

• Rate: The diffusion-rate function, G(t,X_t).

• Alpha: The state vector exponent, which determines the format of D(t,X_t) of G(t,X_t).

• Sigma: The volatility rate, V(t,X_t), of G(t,X_t).

Alpha and Sigma enable you to query the original inputs. (The combined effect of the individual Alpha and Sigma parameters is fully encapsulated by the function stored in Rate.) The Rate functions are the calculation engines for the drift and diffusion objects, and are the only parameters required for simulation.

Note

You can express drift and diffusion classes in the most general form to emphasize the functional (t, X_t) interface. However, you can specify the components A and B as functions that adhere to the common (t, X_t) interface, or as MATLAB arrays of appropriate dimension.

Example: G = diffusion(1, 0.3) % Diffusion rate function G(t,X)

Data Types: struct | double

Object Functions

interpolate	Brownian interpolation of stochastic differential equations
simulate	Simulate multivariate stochastic differential equations (SDEs)
simByEuler	Euler simulation of stochastic differential equations (SDEs)
simByTransition	Simulate Cox-Ingersoll-Ross sample paths with transition density

Examples

Create a cir Object

The Cox-Ingersoll-Ross (CIR) short rate class derives directly from SDE with mean-reverting drift (SDEMRD): $d{X}_t=S(t)[L(t)-X_t]dt+D(t,X_t^{\frac{1}{2}})V(t)dW$

where $$D$$ is a diagonal matrix whose elements are the square root of the corresponding element of the state vector.

Create a cir object to represent the model: $d{X}_t=0.2(0.1-X_t)dt+0.05X_t^{\frac{1}{2}}dW$.

obj = cir(0.2, 0.1, 0.05)  % (Speed, Level, Sigma)

obj = 
   Class CIR: Cox-Ingersoll-Ross
   ----------------------------------------
     Dimensions: State = 1, Brownian = 1
   ----------------------------------------
      StartTime: 0
     StartState: 1
    Correlation: 1
          Drift: drift rate function F(t,X(t)) 
      Diffusion: diffusion rate function G(t,X(t)) 
     Simulation: simulation method/function simByEuler
          Sigma: 0.05
          Level: 0.1
          Speed: 0.2

Although the last two objects are of different classes, they represent the same mathematical model. They differ in that you create the cir object by specifying only three input arguments. This distinction is reinforced by the fact that the Alpha parameter does not display – it is defined to be 1/2.

More About

Instance Hierarchy

There are inheritance relationships among the SDE classes.

The following figure illustrates the inheritance relationships.

For more information, see SDE Class Hierarchy.

Algorithms

When you specify the required input parameters as arrays, they are associated with a specific parametric form. By contrast, when you specify either required input parameter as a function, you can customize virtually any specification.

Accessing the output parameters with no inputs simply returns the original input specification. Thus, when you invoke these parameters with no inputs, they behave like simple properties and allow you to test the data type (double vs. function, or equivalently, static vs. dynamic) of the original input specification. This is useful for validating and designing methods.

When you invoke these parameters with inputs, they behave like functions, giving the impression of dynamic behavior. The parameters accept the observation time t and a state vector X_t, and return an array of appropriate dimension. Even if you originally specified an input as an array, cir treats it as a static function of time and state, by that means guaranteeing that all parameters are accessible by the same interface.

References

[1] Ait-Sahalia, Y. “Testing Continuous-Time Models of the Spot Interest Rate.” The Review of Financial Studies, Spring 1996, Vol. 9, No. 2, pp. 385–426.

[2] Ait-Sahalia, Y. “Transition Densities for Interest Rate and Other Nonlinear Diffusions.” The Journal of Finance, Vol. 54, No. 4, August 1999.

[3] Glasserman, P. Monte Carlo Methods in Financial Engineering. New York, Springer-Verlag, 2004.

[4] Hull, J. C. Options, Futures, and Other Derivatives, 5th ed. Englewood Cliffs, NJ: Prentice Hall, 2002.

[5] Johnson, N. L., S. Kotz, and N. Balakrishnan. Continuous Univariate Distributions. Vol. 2, 2nd ed. New York, John Wiley & Sons, 1995.

[6] Shreve, S. E. Stochastic Calculus for Finance II: Continuous-Time Models. New York: Springer-Verlag, 2004.