In the Trump series 1, 2, 4, …, what is the next term? Is it 8, because the next term is twice the previous term? Is it 7, because the difference between two neighboring terms increases by 1 each time? Or, is it the number 9 (which is based on the formula 1/6(n – 1)(n – 2)(n – 3) + 2ⁿ⁻¹ when n = 4)?