Number of chiral pairs of rows of n colors with exactly 3 different colors.

Terms

a(0) =0a(1) =0a(2) =3a(3) =18a(4) =72a(5) =267a(6) =885a(7) =2880a(8) =9000a(9) =27915a(10) =85233a(11) =259308a(12) =783972a(13) =2366007a(14) =7122405a(15) =21422160a(16) =64364400a(17) =193307955a(18) =580316313a(19) =1741791348a(20) =5226945372a(21) =15684152847a(22) =47058746925a(23) =141189342840a(24) =423593188200