function entropy( k ) {
	var kh;
	if ( isnan( k ) || k <= 0.0 ) {
		return NaN;
	}
	kh = k / 2.0;
	return kh + ln( 2.0*gamma( kh ) ) + ( ( 1.0-kh ) * digamma( kh ) );
}
Returns the differential entropy of a chi-squared distribution.

@param {PositiveNumber} k - degrees of freedom
@returns {number} entropy

@example
var v = entropy( 9.0 );
// returns ~2.786

@example
var v = entropy( 1.0 );
// returns ~0.784

@example
var v = entropy( -0.2 );
// returns NaN

@example
var v = entropy( NaN );
// returns NaN