

A113947


Number of nonequivalent nfold branched coverings of the projective plane with two cyclic branch points.


3



1, 2, 3, 8, 16, 64, 264, 1580, 10648, 84320, 750380, 7455312, 81566928, 974988768, 12636692720, 176505029160, 2642791002368, 42224138928712, 716984262871596, 12893605560786944
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OFFSET

1,2


LINKS



FORMULA

a(n) = (1/n)*Sum_{kn} gcd(2, n/k)*phi(n/k)^2*(n/k)^(k1) * Sum_{i=0..k1} i!*(ki1)! where phi(n) is the Euler function A000010.


MATHEMATICA

a[n_] := 1/n DivisorSum[n, GCD[2, n/#]*EulerPhi[n/#]^2*(n/#)^(#1) Sum[i! * (#i 1)!, {i, 0, #1}]&]; Array[a, 20] (* JeanFrançois Alcover, Oct 05 2016 *)


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



