Which of these is a non-linear recurrence relation?
an=an−1+an−2a_n = a_{n-1} + a_{n-2}an=an−1+an−2
an=3an−1+5a_n = 3a_{n-1} + 5an=3an−1+5
an=an−1+n2a_n = a_{n-1} + n^2an=an−1+n2
an=an−1⋅an−2a_n = a_{n-1} \cdot a_{n-2}an=an−1⋅an−2