function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return pow( v / ( v + pow( x, 2.0 ) ), exponent ) / betaTerm;
	}
Evaluates the probability density function (PDF) for a Student's t distribution.

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

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