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Search: id:A049291
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| A049291 |
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Number of subgroups of index n in free group of rank 4. |
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+0
2
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| 1, 15, 625, 54335, 8563601, 2228419359, 893451975473, 523337983164799, 429463651385469649, 477364501208149290975, 699086688951391180496497, 1318072723102023442664430143, 3137514636520304660660007679505
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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M. Hall, Subgroups of finite index in free groups, Canad. J. Math., 1 (1949), 187-190.
P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 23.
V. A. Liskovets and A. Mednykh, Enumeration of subgroups in the fundamental groups of orientable circle bundles over surfaces, Commun. in Algebra, 28, No. 4 (2000), 1717-1738.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(b).
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FORMULA
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a(n)=n*n!^3-Sum k!^3*a(n-k), k=1..n-1.
L.g.f.: Sum_{n>=1} a(n)*x^n/n = log( Sum_{n>=1} (n-1)!^3*x^n ). [From Paul D. Hanna (pauldhanna(AT)juno.com), Apr 13 2009]
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PROGRAM
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(PARI) {a(n)=n*polcoeff(log(sum(k=0, n, k!^3*x^k)+x*O(x^n)), n)} [From Paul D. Hanna (pauldhanna(AT)juno.com), Apr 13 2009]
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CROSSREFS
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Cf. A003319, A027837, A049290-A049295.
Sequence in context: A027505 A012210 A081022 this_sequence A092958 A079600 A166186
Adjacent sequences: A049288 A049289 A049290 this_sequence A049292 A049293 A049294
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KEYWORD
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easy,nice,nonn
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AUTHOR
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V. A. Liskovets (liskov(AT)im.bas-net.by)
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EXTENSIONS
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More terms from Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), Jun 17 2001
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