# Documentation

## Compute Taylor Series for Univariate Expressions

 Note:   Use only in the MuPAD Notebook Interface. This functionality does not run in MATLAB.

Taylor series expansions serve for approximating an arbitrary expression by a polynomial expression around some value of a variable. Taylor series expansions approximate expressions for which the derivatives up to infinite order exist around a particular value x0 of a variable x:

.

To compute Taylor series expansion, use the taylor command. For example, approximate the expression sin(x)/x around x = 0:

exact := sin(x)/x:
approx := taylor(sin(x)/x, x)

Plot the exact expression and its taylor series expansion in the same coordinate system. The taylor series expansion approximates the expression near x = 0, but visibly deviates from sin(x)/x for larger |x|:

plot(
plot::Function2d(exact, x = -PI..PI,
Legend = "sin(x)/x",
Color = RGB::Red),
plot::Function2d(approx, x = -PI..PI,
Legend = "approximation of sin(x)/x")
)

Accuracy of an approximation depends on the proximity to the expansion point and on the number of terms used in the series expansion. See how to specify the number of terms in Controlling the Number of Terms in Series Expansions.

Taylor series expansions around x = 0 are also called Maclaurin series expansions. Approximate the expressions by Maclaurin series:

taylor(exp(x), x);
taylor(sin(x), x);
taylor(cos(x)/(1 - x), x)

The Maclaurin series expansion does not exist for the following expression. MuPAD® throws an error:

taylor(arccot(x), x)
Error: Cannot compute a Taylor expansion of 'arccot(x)'. Try 'series' for a more general expansion. [taylor]

You can represent the following expression by a Taylor series around x = 1. To compute the series expansion around a nonzero value of a variable, specify the value. For example, compute the Taylor series expansions around x = 1 for the following expressions:

taylor(ln(x), x = 1);
taylor(arccot(x), x = 1)

The taylor command returns results in the form of Taylor series including the order term O. To convert the results to a regular polynomial expression without the O-term, use the expr command:

s := taylor(sin(x)/exp(x), x);
expr(s)