From area of cyclic polygon of 2n + 1 sides.

Terms

a(0) =1a(1) =7a(2) =38a(3) =187a(4) =874a(5) =3958a(6) =17548a(7) =76627a(8) =330818a(9) =1415650a(10) =6015316a(11) =25413342a(12) =106853668a(13) =447472972a(14) =1867450648a(15) =7770342787a(16) =32248174258a(17) =133530264682a(18) =551793690628