# expm1

(PHP 4 >= 4.1.0, PHP 5, PHP 7)

expm1 Returns exp(number) - 1, computed in a way that is accurate even when the value of number is close to zero

### Description

expm1 ( float `\$arg` ) : float

expm1() returns the equivalent to 'exp(`arg`) - 1' computed in a way that is accurate even if the value of `arg` is near zero, a case where 'exp (`arg`) - 1' would be inaccurate due to subtraction of two numbers that are nearly equal.

### Parameters

`arg`

The argument to process

### Return Values

'e' to the power of `arg` minus one

### Changelog

Version Description
5.3.0 This function is now available on all platforms

• log1p() - Returns log(1 + number), computed in a way that is accurate even when the value of number is close to zero
• exp() - Calculates the exponent of e add a note

### User Contributed Notes 2 notes

-1
brettz9 AAT yah
10 years ago
``` Note that exp(x)-1 can be approximated by x + x^2/2! + ... + x^n/n!  and for any value ```
-1
hagen at von-eitzen dot de
16 years ago
``` Compare this to log1p (which is its inverse). Also, You may have to use a similar workaraound in case the underlying C library does not support expm1: <?php function expm1(\$x) {      return (\$x>-1.0e-6 && \$x<1.0e-6) ? (\$x + \$x*\$x/2) : (exp(\$x)-1); } ?> ``` 