1,2

a(n) = (n-1)*(2*n-1)*(2*n-3)!

For example: if n=1 then no polygon exists with 2 different sides. If n=2 then the polygon has 4 different sides A, B, C, D. In this case 3 different types of such 4-angle exist: (A, B, C, D), (A, B, D, C), (A, C, B, D).

Sequence in context: A001084 A137150 A081854 this_sequence A036770 A006821 A165626

Adjacent sequences: A085987 A085988 A085989 this_sequence A085991 A085992 A085993

nonn

Sergey.L.Dolmatov, Almir Dzhumaev (aalma(AT)mail.ru), Aug 18 2003