a(n) = 4*a(n-1) - a(n-2) - 2*(-1)^n, a(0) = 1, a(1) = 4.

Terms

a(0) =1a(1) =4a(2) =13a(3) =50a(4) =185a(5) =692a(6) =2581a(7) =9634a(8) =35953a(9) =134180a(10) =500765a(11) =1868882a(12) =6974761a(13) =26030164a(14) =97145893a(15) =362553410a(16) =1353067745a(17) =5049717572a(18) =18845802541a(19) =70333492594a(20) =262488167833a(21) =979619178740