Number of double tangents of order n.

Terms

a(0) =0a(1) =4a(2) =0a(3) =0a(4) =28a(5) =120a(6) =324a(7) =700a(8) =1320a(9) =2268a(10) =3640a(11) =5544a(12) =8100a(13) =11440a(14) =15708a(15) =21060a(16) =27664a(17) =35700a(18) =45360a(19) =56848a(20) =70380a(21) =86184a(22) =104500a(23) =125580a(24) =149688a(25) =177100a(26) =208104a(27) =243000a(28) =282100a(29) =325728