function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return ( x === mu ) ? PINF : 0.0;
	}
Evaluates the probability density function (PDF) of a degenerate distribution.

@private
@param {number} x - input value
@returns {number} evaluated probability density function

@example
var y = pdf( 10.0 );
// returns <number>