Number of legal 7 X 6 Connect-Four positions after n plies.

Terms

a(0) =1a(1) =7a(2) =49a(3) =238a(4) =1120a(5) =4263a(6) =16422a(7) =54859a(8) =184275a(9) =558186a(10) =1662623a(11) =4568683a(12) =12236101a(13) =30929111a(14) =75437595a(15) =176541259a(16) =394591391a(17) =858218743a(18) =1763883894a(19) =3568259802a(20) =6746155945a(21) =12673345045a(22) =22010823988a(23) =38263228189a(24) =60830813459a(25) =97266114959a(26) =140728569039