function logpdf( 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 : NINF;
	}
	if ( x < 0.0 || x === PINF ) {
		return NINF;
	}
	kh = k / 2.0;
	out = ( ( 1.0-kh ) * LN2 ) + ( ( k-1.0 ) * ln( x ) ) - ( (x*x) / 2.0 );
	out -= gammaln( kh );
	return out;
}
Evaluates the natural logarithm of 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 logPDF

@example
var y = logpdf( 0.3, 4.0 );
// returns ~-4.35

@example
var y = logpdf( 0.7, 0.7 );
// returns ~-0.622

@example
var y = logpdf( -1.0, 0.5 );
// returns -Infinity

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

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

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