function pdf( x ) {
		var xol;
		var z;
		if ( x < 0.0 ) {
			return 0.0;
		}
		if ( x === PINF || x === NINF ) {
			return 0.0;
		}
		if ( x === 0.0 ) {
			return ( k === 1.0 ) ? k / lambda : 0.0;
		}
		xol = x / lambda;
		z = pow( xol, k - 1.0 );
		return ( k / lambda ) * z * exp( -pow( xol, k ) );
	}
Evaluates the probability density function (PDF) for a Weibull distribution.

@private
@param {number} x - input value
@returns {number} evaluated PDF

@example
var y = pdf( 2.3 );
// returns <number>