a(n) = 3*a(n-1) - a(n-2) with a(0)=1, a(1)=7.

Terms

a(0) =1a(1) =7a(2) =20a(3) =53a(4) =139a(5) =364a(6) =953a(7) =2495a(8) =6532a(9) =17101a(10) =44771a(11) =117212a(12) =306865a(13) =803383a(14) =2103284a(15) =5506469a(16) =14416123a(17) =37741900a(18) =98809577a(19) =258686831a(20) =677250916a(21) =1773065917a(22) =4641946835a(23) =12152774588a(24) =31816376929a(25) =83296356199a(26) =218072691668a(27) =570921718805