Which of these recurrence relations is non-linear?
an=an−1+sin(an−2)a_n = a_{n-1} + \sin(a_{n-2})an=an−1+sin(an−2)
an=an−12+1a_n = a_{n-1}^2 + 1an=an−12+1
an=n⋅an−1a_n = n \cdot a_{n-1}an=n⋅an−1
an=an−12+3a_n = \frac{a_{n-1}}{2} + 3an=2an−1+3