Number of acyclic ethylene derivatives with n carbon atoms.

Terms

a(0) =1a(1) =1a(2) =4a(3) =6a(4) =18a(5) =42a(6) =118a(7) =314a(8) =895a(9) =2521a(10) =7307a(11) =21238a(12) =62566a(13) =185310a(14) =553288a(15) =1660490a(16) =5011299a(17) =15190665a(18) =46244031a(19) =141296042a(20) =433204573a(21) =1332261200a(22) =4108833222a(23) =12704949506a(24) =39379343929a(25) =122327632914a(26) =380778749899