a(n) = 5*a(n-2) - 2*a(n-4), n >= 4.

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =23a(5) =46a(6) =105a(7) =210a(8) =479a(9) =958a(10) =2185a(11) =4370a(12) =9967a(13) =19934a(14) =45465a(15) =90930a(16) =207391a(17) =414782a(18) =946025a(19) =1892050a(20) =4315343a(21) =8630686a(22) =19684665a(23) =39369330a(24) =89792639a(25) =179585278a(26) =409593865a(27) =819187730a(28) =1868384047a(29) =3736768094