Number of site swap patterns with 2 balls and exact period n.

Terms

a(0) =1a(1) =2a(2) =6a(3) =15a(4) =42a(5) =107a(6) =294a(7) =780a(8) =2128a(9) =5781a(10) =15918a(11) =43885a(12) =122010a(13) =340323a(14) =954394a(15) =2685930a(16) =7588770a(17) =21507696a(18) =61144062a(19) =174283887a(20) =498012094a(21) =1426213191a(22) =4092816966a(23) =11767176070a(24) =33890202192a(25) =97761428205a(26) =282424564744