Which of the following describes a 'first-order' recurrence relation?
an=an−1+an−2a_n = a_{n-1} + a_{n-2}an=an−1+an−2
an=2an−1a_n = 2a_{n-1}an=2an−1
an=an−1+an−2+an−3a_n = a_{n-1} + a_{n-2} + a_{n-3}an=an−1+an−2+an−3
an=an−1+n2a_n = a_{n-1} + n^2an=an−1+n2