a(n) = A027082(n, n+2).

Terms

a(0) =2a(1) =6a(2) =14a(3) =28a(4) =54a(5) =102a(6) =190a(7) =352a(8) =650a(9) =1198a(10) =2206a(11) =4060a(12) =7470a(13) =13742a(14) =25278a(15) =46496a(16) =85522a(17) =157302a(18) =289326a(19) =532156a(20) =978790a(21) =1800278a(22) =3311230a(23) =6090304a(24) =11201818a(25) =20603358a(26) =37895486a(27) =69700668a(28) =128199518a(29) =235795678