A hierarchical sequence (S(W3{2,2}cc) - see A059126).

Terms

a(0) =18a(1) =90a(2) =270a(3) =666a(4) =1494a(5) =3186a(6) =6606a(7) =13482a(8) =27270a(9) =54882a(10) =110142a(11) =220698a(12) =441846a(13) =884178a(14) =1768878a(15) =3538314a(16) =7077222a(17) =14155074a(18) =28310814a(19) =56622330a(20) =113245398a(21) =226491570a(22) =452983950a(23) =905968746a(24) =1811938374a(25) =3623877666a(26) =7247756286a(27) =14495513562a(28) =28991028150