a(n) = 4*a(n-1) - a(n-2) with a(1) = 1, a(2) = 3.

Terms

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