Which of these is a non-linear recurrence?
an=(an−1)2a_n = (a_{n-1})^2an=(an−1)2
an=2an−1+3a_n = 2a_{n-1} + 3an=2an−1+3
an=an−1+na_n = a_{n-1} + nan=an−1+n
an=an−1−5a_n = a_{n-1} - 5an=an−1−5