function entropy( v ) {
	var out;
	var vh;
	if ( isnan( v ) || v <= 0.0 ) {
		return NaN;
	}
	vh = v / 2.0;
	out = ( v + 1.0 ) / 2.0;
	out *= digamma( ( 1.0+v ) / 2.0 ) - digamma( vh );
	out += ln( sqrt( v ) * beta( vh, 0.5 ) );
	return out;
}
Returns the differential entropy of a Student's t distribution.

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

@example
var v = entropy( 9.0 );
// returns ~1.533

@example
var v = entropy( 2.0 );
// returns ~1.96

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

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