N = 1
                i = n;
                z_val = 20
                val_u = series_sum(L, k, c, a, t, n, z_val)
                while i > 0:
                    if abs(val_u - series_sum(L, k, c, a, t, i, z_val)) > eps:
                        break
                    print(i, abs(val_u - series_sum(L, k, c, a, t, i, z_val)))
                    N = i
                    i = i - 1
                print(N)