a(1)=4, a(2)=6; for n > 2, a(n) = 2*a(n-1) + a(n-2) - 4*((n-1) mod 2).

Terms

a(0) =4a(1) =6a(2) =16a(3) =34a(4) =84a(5) =198a(6) =480a(7) =1154a(8) =2788a(9) =6726a(10) =16240a(11) =39202a(12) =94644a(13) =228486a(14) =551616a(15) =1331714a(16) =3215044a(17) =7761798a(18) =18738640a(19) =45239074a(20) =109216788a(21) =263672646a(22) =636562080a(23) =1536796802a(24) =3710155684a(25) =8957108166a(26) =21624372016a(27) =52205852194a(28) =126036076404a(29) =304278004998