inverf

Inverse of the error function

Use only in the MuPAD Notebook Interface.

This functionality does not run in MATLAB.

Syntax

```inverf(`x`)
```

Description

inverf(x) computes the inverse of the error function.

This function is defined for all complex arguments `x`.

The inverse function `inverf` is singular at the points `x = -1` and `x = 1`.

The inverses of the error functions return floating-point values only for floating-point arguments that belong to a particular interval. Thus, the inverse of error function `inverf(x)` returns floating-point values for real values `x` from the interval `[-1, 1]`. The inverse of the complementary error function `inverfc(x)` returns floating-point values for real values `x` from the interval ```[0, 2]```. The implemented exact values are:

inverf(- 1) = - ∞, inverf(0) = 0, inverf(1) = ∞,

inverfc(0) = ∞, inverfc(1) = 0, inverfc(2) = - ∞.

For all other arguments, the error functions return symbolic function calls.

MuPAD® can simplify expressions that contain error functions and their inverses. For real values `x`, the system applies the following simplification rules:

inverf(erf(x)) = inverf(1 - erfc(x)) = inverfc(1 - erf(x)) = inverfc(erfc(x)) = x,

inverf(- erf(x)) = inverf(erfc(x) - 1) = inverfc(1 + erf(x)) = inverfc(2 - erfc(x)) = - x

For any value `x`, the system applies the following simplification rules:

inverf(- x) = - inverf(x),

inverfc(2 - x) = - inverfc(x),

erf(inverf(x)) = erfc(inverfc(x)) = x.

erf(inverfc(x)) = erfc(inverf(x)) = 1 - x.

Environment Interactions

When called with a floating-point argument, the functions are sensitive to the environment variable `DIGITS`, which determines the numerical working precision.

Examples

Example 1

You can call error functions with exact and symbolic arguments:

`erf(0), erf(3/2), erf(sqrt(2)), erf(infinity)`

`erfc(0), erfc(x + 1), erfc(-infinity)`

`erfc(0, n), erfc(x + 1, -1), erfc(-infinity, 5)`

`erfi(0), erfi(x + 1), erfi(-infinity)`

`inverf(-1), inverf(0), inverf(1), inverf(x + 1), inverf(1/5)`

`inverfc(0), inverfc(1), inverfc(2), inverfc(15), inverfc(x/5)`

For floating-point arguments, the error functions return floating-point values:

`erf(-7.2), erfc(2.0 + 3.5*I), erfc(3.0, 4), erfi(5.5 + 1.0*I)`

For floating-point arguments `x` from the interval ```[-1, 1]```, `inverf` returns floating-point values:

`inverf(-0.5), inverf(0.85)`

For floating-point arguments outside of this interval, `inverf` returns symbolic function calls:

`inverf(-5.3), inverf(10.0)`

For floating-point arguments `x` from the interval ```[0, 2]```, `inverfc` returns floating-point values:

`inverfc(0.5), inverfc(1.25)`

For floating-point arguments outside of this interval, `inverfc` returns symbolic function calls:

`inverfc(-1.25), inverfc(2.5)`

Example 2

For large complex arguments, the error functions can return :

```erf(38000.0 + 3801.0*I), erfi(38000.0 + 3801.0*I), erfc(38000.0 + 3801.0*I)```

For large floating-point arguments with positive real parts, `erfc` can return values truncated to `0.0`:

`erfc(27281.1), erfc(27281.2)`

Example 3

The functions `diff`, `float`, `limit`, `expand`, `rewrite`, and `series` handle expressions involving the error functions:

`diff(erf(x), x, x, x)`

`diff(erfc(x, 3), x, x)`

`diff(inverf(x), x)`

`float(ln(3 + erfi(sqrt(PI)*I)))`

`limit(x/(1 + x)*erf(x), x = infinity)`

`expand(erfc(x, 3))`

`rewrite(inverfc(x), inverf)`

`series(erf(x), x = infinity, 3)`

`series(erfc(x), x = infinity, 3)`

`series(erfi(x), x = I*infinity, 3)`

Parameters

 `x`

Return Values

Arithmetical expression