function entropy( x0, gamma ) {
	if (
		isnan( gamma ) ||
		isnan( x0 ) ||
		gamma <= 0.0
	) {
		return NaN;
	}
	return ln( gamma ) + ln( 4.0*PI );
}
Returns the differential entropy of a Cauchy distribution.

@param {number} x0 - location parameter
@param {PositiveNumber} gamma - scale parameter
@returns {PositiveNumber} entropy

@example
var v = entropy( 10.0, 5.0 );
// returns ~4.14

@example
var v = entropy( 7.0, 0.1 );
// returns ~0.228

@example
var v = entropy( 10.0, -0.5 );
// returns NaN