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A007503
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Number of subgroups of dihedral group: sigma(n) + d(n).
(Formerly M1321)
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37
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2, 5, 6, 10, 8, 16, 10, 19, 16, 22, 14, 34, 16, 28, 28, 36, 20, 45, 22, 48, 36, 40, 26, 68, 34, 46, 44, 62, 32, 80, 34, 69, 52, 58, 52, 100, 40, 64, 60, 98, 44, 104, 46, 90, 84, 76, 50, 134, 60, 99, 76, 104, 56, 128, 76, 128, 84, 94, 62, 180, 64, 100, 110, 134
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OFFSET
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1,1
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COMMENTS
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Write D_{2n} as <a, x | a^n = x^2 = 1, x*a*x = a^(-1)>, then the subgroups are of the form <a^d> for d|n or <a^d, a^r*x> for d|n and 0 <= r < d. There are d(n) subgroups of the first type and sigma(n) subgroups of the second type. - Jianing Song, Jul 21 2022
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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G.f.: Sum_{i>=1} (1+i)*x^i/(1-x^i). - Jon Perry, Jul 03 2004
a(n) = Sum_{d divides n} d+1. - Joerg Arndt, Apr 14 2013
L.g.f.: -log(Product_{k>=1} (1 - x^k)^(1+1/k)) = Sum_{n>=1} a(n)*x^n/n. - Ilya Gutkovskiy, May 23 2018
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EXAMPLE
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a(4) = 10 since D_8 = <a, x | a^4 = x^2 = 1, x*a*x = a^(-1)> has 10 subgroups. The 6 subgroups {e}, {e,a^2}, {e,a,a^2,a^3}, {e,a^2,x,a^2*x}, {e,a^2,a*x,a^3*x} and D_8 are normal, and the 4 subgroups {e,x}, {e,a*x}, {e,a^2*x} and {e,a^3*x} are not. - Jianing Song, Jul 21 2022
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MAPLE
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with(numtheory): seq(sigma(n)+tau(n), n=1..56) ; # Zerinvary Lajos, Jun 04 2008
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MATHEMATICA
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PROG
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(PARI) a(n) = sumdiv(n, d, d+1 ); \\ Joerg Arndt, Apr 14 2013
(Haskell)
a007503 = sum . map (+ 1) . a027750_row'
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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