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

Terms

a(0) =0a(1) =6a(2) =18a(3) =18a(4) =86a(5) =54a(6) =344a(7) =162a(8) =650a(9) =486a(10) =3246a(11) =1458a(12) =11542a(13) =4374a(14) =19196a(15) =13122a(16) =83672a(17) =39366a(18) =434242a(19) =118098a(20) =248278a(21) =354294a(22) =5669112a(23) =1062882a(24) =10522426a(25) =3188646a(26) =5969216a(27) =9565938a(28) =46110338a(29) =28697814