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A049294
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Number of subgroups of index 3 in free group of rank n+1.
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5
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1, 13, 97, 625, 3841, 23233, 139777, 839425, 5038081, 30231553, 181395457, 1088385025, 6530334721, 39182057473, 235092443137, 1410554855425, 8463329525761, 50779977940993, 304679869218817, 1828079218458625
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OFFSET
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0,2
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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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FORMULA
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a(n) = 3*6^n-3*2^n+1.
G.f.: (1+4*x)/((1-x)*(1-2*x)*(1-6*x)). [Colin Barker, May 08 2012]
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MATHEMATICA
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LinearRecurrence[{9, -20, 12}, {1, 13, 97}, 20] (* Harvey P. Dale, Sep 24 2017 *)
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CROSSREFS
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KEYWORD
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easy,nice,nonn
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AUTHOR
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EXTENSIONS
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More terms from Karen Richardson (s1149414(AT)cedarville.edu)
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STATUS
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approved
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