Which of these are valid recursive definitions for the sequence 2,4,8,16,...2, 4, 8, 16, ...2,4,8,16,...?
an=2an−1,a1=2a_n = 2a_{n-1}, a_1 = 2an=2an−1,a1=2
an=an−1+2,a1=2a_n = a_{n-1} + 2, a_1 = 2an=an−1+2,a1=2
an=an−12,a1=2a_n = a_{n-1}^2, a_1 = 2an=an−12,a1=2 (for n>1n>1n>1)
an=2na_n = 2^nan=2n