a(n) = A027113(n, n+2).

Terms

a(0) =3a(1) =9a(2) =20a(3) =40a(4) =77a(5) =145a(6) =270a(7) =500a(8) =923a(9) =1701a(10) =3132a(11) =5764a(12) =10605a(13) =19509a(14) =35886a(15) =66008a(16) =121411a(17) =223313a(18) =410740a(19) =755472a(20) =1389533a(21) =2555753a(22) =4700766a(23) =8646060a(24) =15902587a(25) =29249421a(26) =53798076a(27) =98950092a(28) =181997597a(29) =334745773