function entropy( lambda ) {
	var c;
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	c = -expm1( -lambda );
	return ( lambda * exp( -lambda ) / c ) - ln( c );
}
Returns the differential entropy of a Planck distribution.

@param {PositiveNumber} lambda - shape parameter
@returns {PositiveNumber} differential entropy

@example
var v = entropy( 0.1 );
// returns ~3.3030

@example
var v = entropy( 1.5 );
// returns ~0.6833

@example
var v = entropy( 2.9 );
// returns ~0.2255

@example
var v = entropy( -1.1 );
// returns NaN

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