function entropy( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	return ln( sigma * exp( mu + 0.5 ) * SQRT_TWO_PI );
}
Returns the differential entropy for a lognormal distribution with location `mu` and scale `sigma`.

@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {number} entropy

@example
var y = entropy( 0.0, 1.0 );
// returns ~1.419

@example
var y = entropy( 5.0, 2.0 );
// returns ~7.112

@example
var y = entropy( NaN, 1.0 );
// returns NaN

@example
var y = entropy( 0.0, NaN );
// returns NaN

@example
var y = entropy( 0.0, 0.0 );
// returns NaN