# digamma
> [Digamma][digamma-function] function.
The [digamma function][digamma-function] `ψ` is the logarithmic derivative of the [gamma function][gamma-function], i.e.
## Usage
```javascript
var digamma = require( '@stdlib/math/base/special/digamma' );
```
#### digamma( x )
Evaluates the [digamma function][digamma-function].
```javascript
var v = digamma( -2.5 );
// returns ~1.103
v = digamma( 1.0 );
// returns ~-0.577
v = digamma( 10.0 );
// returns ~2.252
```
If `x` is `0` or a negative `integer`, the function returns `NaN`.
```javascript
var v = digamma( 0.0 );
// returns NaN
v = digamma( -1.0 );
// returns NaN
v = digamma( -2.0 );
// returns NaN
```
If provided `NaN`, the function returns `NaN`.
```javascript
var v = digamma( NaN );
// returns NaN
```
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var digamma = require( '@stdlib/math/base/special/digamma' );
var x;
var v;
var i;
for ( i = 0; i < 10; i++ ) {
x = (randu()*10.0) - 5.0;
v = digamma( x );
console.log( 'x: %d, f(x): %d', x, v );
}
```
[digamma-function]: https://en.wikipedia.org/wiki/Digamma_function
[gamma-function]: https://en.wikipedia.org/wiki/Gamma_function
