Answer :
2**n. (That's two to the nth power: 2 to the first, 2 to the second (or two squared), 2 to the third (or two cubed), 2 to the fourth, etc.)
2**1 = 2 = 2
2**2 = 2 * 2 = 4
2**3 = 2 * 2 * 2 = 8
2**4 = 2 * 2 * 2 * 2 = 16
2**1 = 2 = 2
2**2 = 2 * 2 = 4
2**3 = 2 * 2 * 2 = 8
2**4 = 2 * 2 * 2 * 2 = 16