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A088580 a(n) = 1 + sigma(n). 22
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Number of reflection subgroups of the (dihedral) Coxeter group of type I_2(n).

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

OEIS Wiki, Cyclotomic Polynomials at x=n, n! and sigma(n)

FORMULA

a(n) = 1 + A000203(n).

G.f.: x/(1 - x) + Sum_{k>=1} x^k/(1 - x^k)^2. - Ilya Gutkovskiy, Mar 17 2017

EXAMPLE

a(2)=4. If W=<s, t|s^2=t^2=1, st=ts> then the reflection subgroups are {1}, <s>, <t>, <s, t>.

MAPLE

map(1+numtheory:-sigma, [$1..1000]); # Robert Israel, May 29 2015

MATHEMATICA

Table[1 + DivisorSigma[1, n], {n, 100}] (* Robert Price, May 29 2015 *)

PROG

(Haskell)

a088580 = (+ 1) . a000203  -- Reinhard Zumkeller, Dec 20 2014

(MAGMA) [1+SumOfDivisors(n): n in [1..100]]; // Vincenzo Librandi, May 30 2015

CROSSREFS

Cf. A000203 (sum of divisors of n).

Cf. A065512 (indices of primes in this sequence), A258430 (corresponding primes).

Sequence in context: A340781 A328105 A144831 * A036699 A036694 A085624

Adjacent sequences:  A088577 A088578 A088579 * A088581 A088582 A088583

KEYWORD

easy,nonn

AUTHOR

James East, Nov 20 2003

STATUS

approved

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Last modified June 22 21:23 EDT 2021. Contains 345391 sequences. (Running on oeis4.)