The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 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= then the reflection subgroups are {1}, , , . 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

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