function factory( v ) {
	var exponent;
	var betaTerm;

	if ( isnan( v ) || v <= 0 ) {
		return constantFunction( NaN );
	}
	betaTerm = sqrt( v ) * beta( v/2.0, 0.5 );
	exponent = ( 1.0 + v ) / 2.0;
	return pdf;

	/**
	* 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>
	*/
	function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return pow( v / ( v + pow( x, 2.0 ) ), exponent ) / betaTerm;
	}
}
Returns a function for evaluating the probability density function (PDF) for a Student's t distribution with `v` degrees of freedom.

@param {PositiveNumber} v - degrees of freedom
@returns {Function} PDF

@example
var pdf = factory( 1.0 );
var y = pdf( 3.0 );
// returns ~0.032

y = pdf( 1.0 );
// returns ~0.159