function median( c ) {
	if (
		isnan( c ) ||
		c <= 0.0
	) {
		return NaN;
	}
	return ( sqrt( 1.0 + c ) - 1.0 ) / c;
}
Returns the median of a Bradford distribution.

@param {PositiveNumber} c - shape parameter
@returns {PositiveNumber} median

@example
var v = median( 0.1 );
// returns ~0.488

@example
var v = median( 0.5 );
// returns ~0.449

@example
var v = median( 10.0 );
// returns ~0.232

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

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

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