function factory( k ) {
	var km1;
	var kh;

	if ( isnan( k ) || k < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( k === 0.0 ) {
		return degenerate( 0.0 );
	}

	kh = k / 2.0;
	km1 = k - 1.0;
	return pdf;

	/**
	* 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>
	*/
	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;
	}
}
Returns a function for evaluating the probability density function (PDF) for a chi distribution with degrees of freedom `k`.

@param {NonNegativeNumber} k - degrees of freedom
@returns {Function} PDF

@example
var pdf = factory( 0.5 );

var y = pdf( 2.0 );
// returns ~0.04

y = pdf( 1.0 );
// returns ~0.281