a(n) = (1 - 2*cos(Pi/9))^n + (1 + 2*cos(Pi*2/9))^n + (1 + 2*cos(Pi*4/9))^n.

Terms

a(0) =3a(1) =9a(2) =18a(3) =45a(4) =108a(5) =270a(6) =675a(7) =1701a(8) =4293a(9) =10854a(10) =27459a(11) =69498a(12) =175932a(13) =445419a(14) =1127763a(15) =2855493a(16) =7230222a(17) =18307377a(18) =46355652a(19) =117376290a(20) =297206739a(21) =752553261a(22) =1905530913a(23) =4824972522a(24) =12217257783a(25) =30935180610a(26) =78330624264