What is the 20 th number of this series 3,18,57,132...........????

I don't know if this helps, but if you take the number, subtract the position in the sequence, then raise to the power of 1 divided by three you get a straight line.

Start at the beginning.

Genesis 3.18 - Thorns also and thistles shall it bring forth to thee; and thou shalt eat the herb of the field;

57 - Leads to a variety of bean related things.

132 - 12th Heteromecic number, so may prove useful.

I think the 20th must be 142857, a cyclic number most famous for it's Pie!

namespace aLittleProblem {
    static class Program {
        [STAThread] static void Main() {
            Console.WriteLine("16020");
        }
    }
}

:)

My math is a little rusty (so is my English for that matter) but I think the answer is:
an = 2n^3 + n

Try Here[^]