>895,897
 a_1=3, b_1=1.
 a_{n+1} = 3a_n + 8b_n, b_{n+1} = a_n + 3b_n.
 a_n = {(1+√2)^(2n) + (1-√2)^(2n)}/2, b_n = {(1+√2)^(2n) - (1-√2)^(2n)}/(4√2).