function pdf( x, k ) {
	return gammaPDF( x, k/2.0, 0.5 );
}
Evaluates the probability density function (PDF) for a chi-squared 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 ~0.065

@example
var y = pdf( 0.7, 0.7 );
// returns ~0.274

@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