a(n) = number of reverse alternating fixed-point-free involutions w on 1,2,...,2n, i.e., w(1) < w(2) > w(3) < w(4) > ... < w(2n), w^2=1 and w(i) != i for all i.
A115455
a(n) = number of reverse alternating fixed-point-free involutions w on 1,2,...,2n, i.e., w(1) < w(2) > w(3) < w(4) > ... < w(2n), w^2=1 and w(i) != i for all i.
Terms
- a(0) =1a(1) =0a(2) =1a(3) =1a(4) =4a(5) =13a(6) =59a(7) =308a(8) =1871a(9) =12879a(10) =99144a(11) =843735a(12) =7865177a(13) =79698760a(14) =872235089a(15) =10253148625a(16) =128839087676
External references
- oeis: A115455