a(n) = 6*a(n-2) + a(n-4), where a(0) = 3, a(1) = 11, a(2) = 18, a(3) = 68.

Terms

a(0) =3a(1) =11a(2) =18a(3) =68a(4) =111a(5) =419a(6) =684a(7) =2582a(8) =4215a(9) =15911a(10) =25974a(11) =98048a(12) =160059a(13) =604199a(14) =986328a(15) =3723242a(16) =6078027a(17) =22943651a(18) =37454490a(19) =141385148a(20) =230804967a(21) =871254539a(22) =1422284292a(23) =5368912382a(24) =8764510719a(25) =33084728831a(26) =54009348606a(27) =203877285368a(28) =332820602355