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

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

@example
var v = variance( 0.1 );
// returns ~0.083

@example
var v = variance( 0.5 );
// returns ~0.083

@example
var v = variance( 10.0 );
// returns ~0.076

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

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

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