function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a ) {
			return 0.0;
		}
		// Case: x >= a
		if ( x < c ) {
			return 2.0 * ( x - a ) / denom1;
		}
		if ( x === c ) {
			return 2.0 / denom2;
		}
		// Case: x > c
		if ( x <= b ) {
			return 2.0 * ( b - x ) / denom3;
		}
		// Case: x > b
		return 0.0;
	}
Evaluates the probability density function (PDF) for a triangular distribution.

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

@example
var y = pdf( 12.0 );
// returns <number>