function logpdf( x ) {
		var out;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 || x === PINF ) {
			return NINF;
		}
		out = ( ( 1.0-kh ) * LN2 ) + ( km1 * 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`.

@private
@param {number} x - input value
@returns {number} evaluated logPDF

@example
var y = logpdf( 1.0 );
// returns <number>