a(n) = 3*a(n-1) - a(n-2) with a(0)=1, a(1)=9.

Terms

a(0) =1a(1) =9a(2) =26a(3) =69a(4) =181a(5) =474a(6) =1241a(7) =3249a(8) =8506a(9) =22269a(10) =58301a(11) =152634a(12) =399601a(13) =1046169a(14) =2738906a(15) =7170549a(16) =18772741a(17) =49147674a(18) =128670281a(19) =336863169a(20) =881919226a(21) =2308894509a(22) =6044764301a(23) =15825398394a(24) =41431430881a(25) =108468894249