Number of spiro bicyclic skeletons with n carbon atoms.

Terms

a(0) =1a(1) =4a(2) =17a(3) =60a(4) =209a(5) =685a(6) =2204a(7) =6913a(8) =21387a(9) =65241a(10) =197104a(11) =590284a(12) =1755804a(13) =5191723a(14) =15276610a(15) =44761350a(16) =130682371a(17) =380337474a(18) =1103927291a(19) =3196494453