Sum of terms in periodic part of continued fraction expansion of square root of -1 + 3^n.

Terms

a(0) =2a(1) =5a(2) =10a(3) =17a(4) =56a(5) =53a(6) =160a(7) =161a(8) =346a(9) =485a(10) =1850a(11) =1457a(12) =3764a(13) =4373a(14) =13468a(15) =13121a(16) =43572a(17) =39365a(18) =192642a(19) =118097a(20) =226348a(21) =354293a(22) =2646006a(23) =1062881a(24) =1871694a(25) =3188645a(26) =5646564a(27) =9565937a(28) =36393508a(29) =28697813