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A113947
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Number of non-equivalent n-fold branched coverings of the projective plane with two cyclic branch points.
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2
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1, 2, 3, 8, 16, 64, 264, 1580, 10648, 84320, 750380, 7455312, 81566928, 974988768, 12636692720, 176505029160, 2642791002368, 42224138928712, 716984262871596, 12893605560786944
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| J. H. Kwak, A. Mednykh and V. Liskovets, Enumeration of branched coverings of nonorientable surfaces with cyclic branch points, SIAM J. Discrete Math., Vol. 19, No. 2 (2005), 388-398.
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FORMULA
| a(n)=(1/n)sum_{k|n}GCD(2, n/k)*phi(n/k)^2*(n/k)^(k-1)sum_{i=0..k-1}i!*(k-i-1)! where phi(n) is the Euler function A000010.
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CROSSREFS
| Cf. A113948, A113949.
Sequence in context: A204516 A185171 A005648 * A102008 A200083 A158921
Adjacent sequences: A113944 A113945 A113946 * A113948 A113949 A113950
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KEYWORD
| nonn
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AUTHOR
| Valery A. Liskovets (liskov(AT)im.bas-net.by), Nov 10 2005
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