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

Terms

a(0) =1a(1) =1a(2) =3a(3) =11a(4) =41a(5) =153a(6) =571a(7) =2131a(8) =7953a(9) =29681a(10) =110771a(11) =413403a(12) =1542841a(13) =5757961a(14) =21489003a(15) =80198051a(16) =299303201a(17) =1117014753a(18) =4168755811a(19) =15558008491a(20) =58063278153a(21) =216695104121a(22) =808717138331