function pdf( x, c ) {
	var k;
	if (
		isnan( c ) ||
		isnan( x ) ||
		c <= 0.0
	) {
		return NaN;
	}
	if ( x < 0.0 || x > 1.0 ) {
		// Support of the Bradford distribution: [0,1]
		return 0.0;
	}
	k = log1p( c );
	return c / ( ( 1.0 + ( c*x ) ) * k );
}
Evaluates the probability density function (PDF) for a Bradford distribution with shape parameter `c` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} c - shape parameter
@returns {Probability} evaluated PDF

@example
var v = pdf( 0.1, 0.1 );
// returns ~1.039

@example
var v = pdf( 0.5, 5.0 );
// returns ~0.797

@example
var v = pdf( 1.0, 10.0 );
// returns ~0.379

@example
var v = pdf( 2.0, 0.5 );
// returns 0.0

@example
var v = pdf( -1.0, 0.5 );
// returns 0.0

@example
var v = pdf( 0.5, 0.0 );
// returns NaN

@example
var v = pdf( 0.5, -5.0 );
// returns NaN

@example
var v = pdf( NaN, 1.0 );
// returns NaN

@example
var v = pdf( 1.0, NaN );
// returns NaN