Number of trisubstituted alkanes C_n H_{2n-1} X_2 Y with n carbon atoms.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =29a(5) =91a(6) =282a(7) =864a(8) =2624a(9) =7905a(10) =23665a(11) =70458a(12) =208813a(13) =616400a(14) =1813354a(15) =5318673a(16) =15559072a(17) =45410239a(18) =132258752a(19) =384493609a(20) =1115907585a(21) =3233782193a(22) =9358293913a(23) =27048189854a(24) =78087437216a(25) =225198736949a(26) =648826102943