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A088580
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a(n) = 1 + sigma(n).
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23
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2, 4, 5, 8, 7, 13, 9, 16, 14, 19, 13, 29, 15, 25, 25, 32, 19, 40, 21, 43, 33, 37, 25, 61, 32, 43, 41, 57, 31, 73, 33, 64, 49, 55, 49, 92, 39, 61, 57, 91, 43, 97, 45, 85, 79, 73, 49, 125, 58, 94, 73, 99, 55, 121, 73, 121, 81, 91, 61, 169, 63, 97, 105, 128, 85, 145, 69, 127, 97
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OFFSET
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1,1
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COMMENTS
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Number of reflection subgroups of the (dihedral) Coxeter group of type I_2(n).
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LINKS
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FORMULA
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G.f.: x/(1 - x) + Sum_{k>=1} x^k/(1 - x^k)^2. - Ilya Gutkovskiy, Mar 17 2017
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EXAMPLE
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a(2)=4. If W=<s, t|s^2=t^2=1, st=ts> then the reflection subgroups are {1}, <s>, <t>, <s, t>.
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MAPLE
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map(1+numtheory:-sigma, [$1..1000]); # Robert Israel, May 29 2015
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MATHEMATICA
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Table[1 + DivisorSigma[1, n], {n, 100}] (* Robert Price, May 29 2015 *)
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PROG
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(Haskell)
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CROSSREFS
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Cf. A000203 (sum of divisors of n).
Cf. A065512 (indices of primes in this sequence), A258430 (corresponding primes).
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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