Number of site swap patterns with 3 balls and exact period n.

Terms

a(0) =1a(1) =3a(2) =12a(3) =42a(4) =156a(5) =554a(6) =2028a(7) =7350a(8) =26936a(9) =98874a(10) =365196a(11) =1353520a(12) =5039580a(13) =18831306a(14) =70626140a(15) =265741350a(16) =1002984060a(17) =3796211692a(18) =14406086604a(19) =54801192684a(20) =208932673508a(21) =798218225802