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

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

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