A362819 Number of ordered pairs of involutions on [n] that commute.
1, 1, 4, 10, 52, 196, 1216, 5944, 42400, 250912, 2008576, 13815616, 122074624, 950640640, 9158267392, 79258479616, 824644235776, 7823203807744, 87245790791680, 897748312609792, 10665239974537216, 118040852776093696, 1486172381689544704, 17572063073426206720, 233446797379437248512
Offset: 0
Keywords
Links
- Andrew Howroyd, Table of n, a(n) for n = 0..200
Crossrefs
Programs
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PARI
b(n,f) = {sum(k=0, n\2, f(k)*binomial(n,2*k)*(2*k)!/(k!*2^k))} a(n) = {b(n, k->b(n-2*k, j->1)*b(k, j->2^(k-j)))} -
PARI
seq(n)=Vec(serlaplace(exp(x + 3*x^2/2 + x^4/4 + O(x*x^n))))
Comments