function pdf( x ) {
		var out;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		out = pow( 2.0, 1.0-kh ) * pow( x, km1 ) * 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`.

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

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