function factory( v ) {
	if ( isnan( v ) || v <= 0.0 ) {
		return constantFunction( NaN );
	}
	return quantile;

	/**
	* Evaluates the quantile function for a Student's t distribution.
	*
	* @private
	* @param {Probability} p - input value
	* @returns {number} evaluated quantile function
	*
	* @example
	* var y = quantile( 0.3 );
	* // returns <number>
	*/
	function quantile( p ) {
		var prob;
		var xs;

		if ( isnan( p ) || p < 0.0 || p > 1.0 ) {
			return NaN;
		}
		prob = ( p > 0.5 ) ? 1.0 - p : p;
		xs = kernelBetaincinv( v / 2.0, 0.5, 2.0 * prob, 1.0 - (2.0 * prob) );
		return sign( p - 0.5 ) * sqrt( v * xs[ 1 ] / xs[ 0 ] );
	}
}
Returns a function for evaluating the quantile function for a Student's t distribution with degrees of freedom `v`.

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

@example
var quantile = factory( 0.5 );
var y = quantile( 0.5 );
// returns 0.0

y = quantile( 0.8 );
// returns ~2.513

y = quantile( 1.0 );
// returns Infinity