a(1) = 2, a(2) = 8; a(n) = 2 a(n - 1) + a(n - 2) - 4*(n mod 2).

Terms

a(0) =2a(1) =8a(2) =14a(3) =36a(4) =82a(5) =200a(6) =478a(7) =1156a(8) =2786a(9) =6728a(10) =16238a(11) =39204a(12) =94642a(13) =228488a(14) =551614a(15) =1331716a(16) =3215042a(17) =7761800a(18) =18738638a(19) =45239076a(20) =109216786a(21) =263672648a(22) =636562078a(23) =1536796804a(24) =3710155682a(25) =8957108168a(26) =21624372014a(27) =52205852196a(28) =126036076402a(29) =304278005000