function lowerGammaSeries( a, z, initialValue ) {
	var result;
	var s;

	initialValue = initialValue || 0.0;
	s = lowerIncompleteGammaSeries( a, z );
	opts.initialValue = initialValue;
	result = sumSeries( s, opts );
	return result;
}
Sums elements of the series expansion of the lower incomplete gamma function.

## Method

-   Multiply result by `(z^a)*(e^-z) / a` to get the full lower incomplete integral.
-   Divide by `tgamma(a)` to get the normalized value.

@private
@param {number} a - function parameter
@param {number} z - function parameter
@param {number} initialValue - initial value of the resulting sum
@returns {number} sum of terms of lower gamma series