Which of the following is a non-linear recurrence relation?
an=5an−1a_n = 5a_{n-1}an=5an−1
an=an−1+2na_n = a_{n-1} + 2nan=an−1+2n
an=(an−1)2a_n = (a_{n-1})^2an=(an−1)2
an=2an−1−an−2a_n = 2a_{n-1} - a_{n-2}an=2an−1−an−2