a(n) = 3*a(n-1) + a(n-3) with a(0)=1, a(1)=2, a(2)=6.

Terms

a(0) =1a(1) =2a(2) =6a(3) =19a(4) =59a(5) =183a(6) =568a(7) =1763a(8) =5472a(9) =16984a(10) =52715a(11) =163617a(12) =507835a(13) =1576220a(14) =4892277a(15) =15184666a(16) =47130218a(17) =146282931a(18) =454033459a(19) =1409230595a(20) =4373974716a(21) =13575957607a(22) =42137103416a(23) =130785284964a(24) =405931812499