a(n) is the number of symmetrical linear hydrocarbon chains with n C-C bonds.

Terms

a(0) =1a(1) =3a(2) =2a(3) =6a(4) =5a(5) =14a(6) =11a(7) =31a(8) =25a(9) =70a(10) =56a(11) =157a(12) =126a(13) =353a(14) =283a(15) =793a(16) =636a(17) =1782a(18) =1429a(19) =4004a(20) =3211a(21) =8997a(22) =7215a(23) =20216a(24) =16212a(25) =45425a(26) =36428a(27) =102069a(28) =81853a(29) =229347