function lowerIncompleteGammaSeries( a1, z1 ) {
	var result = 1.0;
	var a = a1;
	var z = z1;
	return next;

	/**
	* Calculate the next term of the series.
	*
	* @private
	* @returns {number} series expansion term
	*/
	function next() {
		var r = result;
		a += 1.0;
		result *= z/a;
		return r;
	}
}
Creates a function to evaluate a series expansion of the incomplete gamma function.

@private
@param {number} a1 - function parameter
@param {number} z1 - function parameter
@returns {Function} series function