function logpdf( x, v ) {
	var betaTerm;
	if (
		isnan( x ) ||
		isnan( v ) ||
		v <= 0.0
	) {
		return NaN;
	}
	betaTerm = ln( sqrt( v ) ) + betaln( v/2.0, 0.5 );
	return ( ( (1.0+v) / 2.0 ) * ln( v / ( v + pow( x, 2.0 ) ) ) ) - betaTerm;
}
Evaluates the natural logarithm of the probability density function (PDF) for a Student's t distribution with degrees of freedom `v` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} v - degrees of freedom
@returns {number} evaluated logPDF

@example
var y = logpdf( 0.3, 4.0 );
// returns ~-1.036

@example
var y = logpdf( 2.0, 0.7 );
// returns ~-2.841

@example
var y = logpdf( -1.0, 0.5 );
// returns ~-2.134

@example
var y = logpdf( 0.0, NaN );
// returns NaN

@example
var y = logpdf( NaN, 2.0 );
// returns NaN

@example
var y = logpdf( 2.0, -1.0 );
// returns NaN