function logpdf( x, k ) {
	return gammaLogPDF( x, k/2.0, 0.5 );
}
Evaluates the natural logarithm of 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 logPDF

@example
var y = logpdf( 0.3, 4.0 );
// returns ~-2.74

@example
var y = logpdf( 0.7, 0.7 );
// returns ~-1.295

@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