vpaintegral

Numerical integration using variable precision

Syntax

vpaintegral(f,a,b)

vpaintegral(f,x,a,b)

vpaintegral(___,Name,Value)

Description

vpaintegral(f,a,b) numerically approximates f from a to b. The default variable x in f is found by symvar.

vpaintegral(f,[a b]) is equal to vpaintegral(f,a,b).

vpaintegral(f,x,a,b) performs numerical integration using the integration variable x.

vpaintegral(___,Name,Value) uses additional options specified by one or more Name,Value pair arguments.

Examples

Numerically Integrate Symbolic Expression

Numerically integrate the symbolic expression x^2 from 1 to 2.

syms x
vpaintegral(x^2, 1, 2)

ans =
2.33333

Numerically Integrate Symbolic Function

Numerically integrate the symbolic function y(x) = x² from 1 to 2.

syms y(x)
y(x) = x^2;
vpaintegral(y, 1, 2)

ans =
2.33333

High-Precision Numerical Integration

vpaintegral uses variable-precision arithmetic while the MATLAB^® integral function uses double-precision arithmetic. Using the default values of tolerance, vpaintegral can handle values that cause the MATLAB integral function to overflow or underflow.

Integrate besseli(5,25*u).*exp(-u*25) by using both integral and vpaintegral. The integral function returns NaN and issues a warning while vpaintegral returns the correct result.

syms u x
f = besseli(5,25*x).*exp(-x*25);
fun = @(u)besseli(5,25*u).*exp(-u*25);

usingIntegral = integral(fun, 0, 30)
usingVpaintegral = vpaintegral(f, 0, 30)

Warning: Infinite or Not-a-Number value encountered. 
usingIntegral =
   NaN

usingVpaintegral =
0.688424

Increase Precision Using Tolerances

The digits function does not affect vpaintegral. Instead, increase the precision of vpainteral by decreasing the integration tolerances. Conversely, increase the speed of numerical integration by increasing the tolerances. Control the tolerance used by vpaintegral by changing the relative tolerance RelTol and absolute tolerance AbsTol, which affect the integration through the condition

$$\begin{array}{l}\left|Q-I\right|\le \text{max}(AbsTol,\left|Q\right|·RelTol)\\ \begin{array}{ccc}\text{where}& Q=\text{Calculated Integral}& \\ & I=\text{Exact Integral}\text{.}& \end{array}\end{array}$$

Numerically integrate besselj(0,x) from 0 to pi, to 32 significant figures by setting RelTol to 10^(-32). Turn off AbsTol by setting it to 0.

syms x
vpaintegral(besselj(0,x), [0 pi], 'RelTol', 1e-32, 'AbsTol', 0)

ans =
1.3475263146739901712314731279612

Using lower tolerance values increases precision at the cost of speed.

Complex Path Integration Using Waypoints

Integrate 1/(2*z-1) over the triangular path from 0 to 1+1i to 1-1i back to 0 by specifying waypoints.

syms z
vpaintegral(1/(2*z-1), [0 0], 'Waypoints', [1+1i 1-1i])

ans =
- 8.67362e-19 - 3.14159i

Reversing the direction of the integral, by changing the order of the waypoints and exchanging the limits, changes the sign of the result.

Multiple Integrals

Perform multiple integration by nesting calls to vpaintegral. Integrate

$${\displaystyle \underset{-1}{\overset{2}{\int }}{\displaystyle \underset{1}{\overset{3}{\int }}xydxdy}}.$$

syms x y
vpaintegral(vpaintegral(x*y, x, [1 3]), y, [-1 2])

ans =
6.0

The limits of integration can be symbolic expressions or functions. Integrate over the triangular region 0 ≤ x ≤ 1 and |y| < x by specifying the limits of the integration over y in terms of x.

vpaintegral(vpaintegral(sin(x-y)/(x-y), y, [-x x]), x, [0 1])

ans =
0.89734

Input Arguments

f — Expression or function to integrate

symbolic number | symbolic variable | symbolic vector | symbolic matrix | symbolic multidimensional array | symbolic function | symbolic expression

Expression or function to integrate, specified as a symbolic number, variable, vector, matrix, multidimensional array, function, or expression.

a,b — Limits of integration

list of two numbers | list of two symbolic numbers | list of two symbolic variables | list of two symbolic functions | list of two symbolic expressions

Limits of integration, specified as a list of two numbers, symbolic numbers, symbolic variables, symbolic functions, or symbolic expressions.

x — Integration variable

symbolic variable

Integration variable, specified as a symbolic variable. If x is not specified, the integration variable is found by symvar.

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: 'RelTol',1e-20

'RelTol' — Relative error tolerance

1e-6 (default) | positive real number

Relative error tolerance, specified as a positive real number. The default is 1e-6. The RelTol argument determines the accuracy of the integration only if $$RelTol·\left|Q\right|>AbsTol$$, where Q is the calculated integral. In this case, vpaintegral satisfies the condition $$\left|Q-I\right|\le RelTol·\left|Q\right|$$, where I is the exact integral value. To use only RelTol and turn off AbsTol, set AbsTol to 0.

Example: 1e-8

'AbsTol' — Absolute error tolerance

1e-10 (default) | non-negative real number

Absolute error tolerance, specified as a non-negative real number. The default is 1e-10. AbsTol determines the accuracy of the integration if $$AbsTol>RelTol·\left|Q\right|$$, where Q is the calculated integral. In this case, vpaintegral satisfies the condition $$\left|Q-I\right|\le AbsTol$$, where I is the exact integral value. To turn off AbsTol and use only RelTol, set AbsTol to 0.

Example: 1e-12

'Waypoints' — Integration path

vector of numbers | vector of symbolic numbers | vector of symbolic expressions | vector of symbolic functions

Integration path, specified as a vector of numbers, or as a vector of symbolic numbers, expressions, or functions. vpaintegral integrates along the sequence of straight-line paths (lower limit to the first waypoint, from the first to the second waypoint, and so on) and finally from the last waypoint to the upper limit. For contour integrals, set equal lower and upper limits and define the contour using waypoints.

'MaxFunctionCalls' — Maximum evaluations of input

10^5 (default) | positive integer | positive symbolic integer

Maximum evaluations of input, specified as a positive integer or a positive symbolic integer. The default value is 10^5. If the number of evaluations of f is greater than MaxFunctionCalls, then vpaintegral throws an error. For unlimited evaluations, set MaxFunctionCalls to Inf.

Tips

• Ensure that the input is integrable. If the input is not integrable, the output of vpaintegral is unpredictable.

• The digits function does not affect vpaintegral. To increase precision, use the RelTol and AbsTol arguments instead.