function pdf( x, k ) {
	var out;
	var kh;
	if (
		isnan( x ) ||
		isnan( k ) ||
		k < 0.0
	) {
		return NaN;
	}
	if ( k === 0.0 ) {
		// Point mass at 0...
		return ( x === 0.0 ) ? PINF : 0.0;
	}
	if ( x < 0.0 ) {
		return 0.0;
	}
	kh = k / 2.0;
	out = pow( 2.0, 1.0-kh ) * pow( x, k-1.0 ) * exp( -(x*x)/2.0 );
	out /= gamma( kh );
	return out;
}
Evaluates the probability density function (PDF) for a chi distribution with degrees of freedom `k` at a value `x`.

@param {number} x - input value
@param {NonNegativeNumber} k - degrees of freedom
@returns {number} evaluated PDF

@example
var y = pdf( 0.3, 4.0 );
// returns ~0.013

@example
var y = pdf( 0.7, 0.7 );
// returns ~0.537

@example
var y = pdf( -1.0, 0.5 );
// returns 0.0

@example
var y = pdf( 0.0, NaN );
// returns NaN

@example
var y = pdf( NaN, 2.0 );
// returns NaN

@example
// Negative degrees of freedom:
var y = pdf( 2.0, -1.0 );
// returns NaN