The recurrence an=2an−1a_n = 2a_{n-1}an=2an−1 with a1=3a_1 = 3a1=3 has closed-form solution an=a_n =an=:
3⋅2n−13 \cdot 2^{n-1}3⋅2n−1
2⋅3n−12 \cdot 3^{n-1}2⋅3n−1
3n3n3n
2n+12^n + 12n+1