static double SumGeometricElements(int a1, int alim, double t)

{

    if (t < 0 || t > 1)

        throw new ArgumentException("Progression step cannot be less than zero or greater than one");

​

    if (a1 <= alim)

        return 0;

​

    double result = a1;

    double lastElement = a1;

​

    while (lastElement * t > alim)

    {

        lastElement *= t;

        result += lastElement;

    }

    return result;

}