OFFSET
1,1
COMMENTS
Number of cyclic subgroups of dihedral group with 2n elements.
a(n) is the n-th smallest number not a divisor of n. - J. Lowell, Apr 06 2008
LINKS
Harry J. Smith, Table of n, a(n) for n=1..1000
FORMULA
a(n) = n + A000005(n). - Omar E. Pol, Dec 12 2008
From Ilya Gutkovskiy, Apr 12 2017: (Start)
G.f.: x/(1 - x)^2 + Sum_{k>=1} x^k/(1 - x^k).
Dirichlet g.f.: zeta(s)^2 + zeta(s-1). (End)
MAPLE
with(numtheory):seq(n+tau(n), n=1..71) ; # Zerinvary Lajos, Jun 04 2008
MATHEMATICA
Table[n + DivisorSigma[0, n], {n, 100}] (* Indranil Ghosh, Apr 12 2017 *)
PROG
(PARI) a(n) = n + numdiv(n) \\ Harry J. Smith, Aug 03 2009
(Haskell)
a062249 n = a000005 n + n -- Reinhard Zumkeller, Mar 29 2014
(Python)
from sympy.ntheory import divisor_count
[n + divisor_count(n) for n in range(101)] # Indranil Ghosh, Apr 12 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ahmed Fares (ahmedfares(AT)my-deja.com), Jul 01 2001
EXTENSIONS
Formula and more terms from Vladeta Jovovic, Jul 03 2001
STATUS
approved