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Search: id:A049295
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| A049295 |
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Number of subgroups of index 4 in free group of rank n+1. |
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+0
7
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| 1, 71, 2143, 54335, 1321471, 31817471, 764217343, 31817471, 764217343, 18344733695, 440294408191, 10567189327871, 253613279903743, 6086723113107455, 146081381003558911, 3505953301484470271, 84142880178680889343, 2019429129941297135615
(list; graph; listen)
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OFFSET
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0,2
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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.
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)=4*24^n-4*6^n-2*4^n+4*2^n-1.
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CROSSREFS
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Cf. A003319, A027837, A049290-A049294.
Sequence in context: A050885 A071827 A093271 this_sequence A017787 A017734 A115447
Adjacent sequences: A049292 A049293 A049294 this_sequence A049296 A049297 A049298
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KEYWORD
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easy,nonn,nice
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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 Carrie Westbrook (s1213407(AT)cedarville.edu)
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