# Trigamma
> [Trigamma][trigamma-function] function.
The [trigamma function][trigamma-function] `ψ^(1)` is the derivative of the [digamma function][@stdlib/math/base/special/digamma].
## Usage
```javascript
var trigamma = require( '@stdlib/math/base/special/trigamma' );
```
#### trigamma( x )
Evaluates the [trigamma function][trigamma-function].
```javascript
var v = trigamma( -2.5 );
// returns ~9.539
v = trigamma( 1.0 );
// returns ~1.645
v = trigamma( 10.0 );
// returns ~0.105
```
If `x` is `0` or a negative `integer`, the function returns `NaN`.
```javascript
var v = trigamma( 0.0 );
// returns NaN
v = trigamma( -1.0 );
// returns NaN
v = trigamma( -2.0 );
// returns NaN
```
If provided `NaN`, the function returns `NaN`.
```javascript
var v = trigamma( NaN );
// returns NaN
```
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var trigamma = require( '@stdlib/math/base/special/trigamma' );
var x;
var v;
var i;
for ( i = 0; i < 10; i++ ) {
x = (randu()*100.0) - 50.0;
v = trigamma( x );
console.log( 'x: %d, ψ^(1)(x): %d', x, v );
}
```
[trigamma-function]: https://en.wikipedia.org/wiki/Trigamma_function
[@stdlib/math/base/special/digamma]: https://www.npmjs.com/package/@stdlib/math/tree/main/base/special/digamma
