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A057014
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Number of subgroups of index n in free group of rank n.
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1
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1, 3, 97, 54335, 1035045121, 1160799620549247, 114730150164000898447873, 1385904943637483363530115691960319, 2706118201409447403878672281480056877837885441, 1091106883064604841286281006789753438364086962298324514658303
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OFFSET
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1,2
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REFERENCES
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P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 23.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(b).
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LINKS
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MATHEMATICA
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t[n_, k_] := t[n, k] = k*k!^(n - 1) - Sum[j!^(n - 1)*t[n, k - j], {j, 1, k - 1}];
a[n_] := t[n, n];
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001
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STATUS
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approved
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