a(n) = 3*a(n-1)+a(n-2)-a(n-3)+a(n-6)-a(n-7)+a(n-10)-a(n-11).

Terms

a(0) =2a(1) =6a(2) =19a(3) =61a(4) =196a(5) =630a(6) =2026a(7) =6516a(8) =20957a(9) =67403a(10) =216786a(11) =697242a(12) =2242518a(13) =7212542a(14) =23197478a(15) =74609338a(16) =239963729a(17) =771787993a(18) =2482278087a(19) =7983674995a(20) =25677649398a(21) =82586237418a(22) =265619586324a(23) =854304141280