Number of paraffins C_n H_{2n} X_2 with n carbon atoms.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =21a(5) =52a(6) =129a(7) =332a(8) =859a(9) =2261a(10) =5983a(11) =15976a(12) =42836a(13) =115469a(14) =312246a(15) =847241a(16) =2304522a(17) =6283327a(18) =17164401a(19) =46972357a(20) =128741107a(21) =353345434a(22) =970999198a(23) =2671347292a(24) =7356752678a(25) =20279171785a(26) =55948407837a(27) =154479213626a(28) =426845422807