function entropy( k ) {
	var kh;
	if ( isnan( k ) || k <= 0.0 ) {
		return NaN;
	}
	kh = k / 2.0;
	return gammaln( kh ) + ( 0.5 * ( k - LN2 - ( ( k-1.0 )*digamma( kh ) ) ) );
}
Returns the differential entropy of a chi distribution.

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

@example
var v = entropy( 9.0 );
// returns ~1.052

@example
var v = entropy( 1.0 );
// returns ~0.726

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

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