function entropy( c ) {
	var k;
	if (
		isnan( c ) ||
		c <= 0.0
	) {
		return NaN;
	}
	k = ln( 1.0 + c );
	return ( k/2.0 ) - ln( c/k );
}
Returns the differential entropy of a Bradford distribution.

@param {PositiveNumber} c - shape parameter
@returns {number} differential entropy

@example
var v = entropy( 0.2 );
// returns ~-0.001

@example
var v = entropy( 0.5 );
// returns ~-0.007

@example
var v = entropy( 10.0 );
// returns ~-0.229

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

@example
var v = entropy( -1.0 );
// returns NaN

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