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A062011 a(n) = 2*tau(n) = 2*A000005(n). 13
2, 4, 4, 6, 4, 8, 4, 8, 6, 8, 4, 12, 4, 8, 8, 10, 4, 12, 4, 12, 8, 8, 4, 16, 6, 8, 8, 12, 4, 16, 4, 12, 8, 8, 8, 18, 4, 8, 8, 16, 4, 16, 4, 12, 12, 8, 4, 20, 6, 12, 8, 12, 4, 16, 8, 16, 8, 8, 4, 24, 4, 8, 12, 14, 8, 16, 4, 12, 8, 16, 4, 24, 4, 8, 12, 12, 8, 16, 4, 20, 10, 8, 4, 24, 8, 8, 8, 16 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Old definition was "Number of cyclic subgroups of the group C_n X C_2 (where C_n is the cyclic group with n elements)."

More generally, the number of cyclic subgroups of the group C_n X C_m is Sum_{i|n, j|m} phi(i)*phi(j)/phi(lcm(i,j)), where phi=Euler totient function, cf. A000010. - Vladeta Jovovic, Jul 15 2001

Also number of divisors of p*n, where p is any prime not dividing n, e.g.: a(n) = A000005(A087560(n)) = A000005(A119416(n)). - Reinhard Zumkeller, May 17 2006

If p(x) is a polynomial with integer coefficients, and if r is an integer zero of p(x), then r is a divisor of the constant term c_0 of p(x). Under this theorem, p(x) can have a(c_0) possible integer roots. a(n) is also the number of integer divisor of n, while A000005(n) is the number of positive divisors. - Enrique Pérez Herrero, Jul 21 2011

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..1000

Omar E. Pol, Illustration of initial terms

Index entries for sequences related to groups

FORMULA

L.g.f.: -log(Product_{k>=1} (1 - x^k)^(2/k)) = Sum_{n>=1} a(n)*x^n/n. - Ilya Gutkovskiy, Mar 18 2018

MATHEMATICA

A062011[n_] := 2*DivisorSigma[0, n]; Array[A062011, 50] (* Enrique Pérez Herrero, Jul 15 2011 *)

PROG

(PARI) for (n=1, 1000, write("b062011.txt", n, " ", 2*numdiv(n))) \\ Harry J. Smith, Jul 29 2009

CROSSREFS

Cf. A000005, A060648, A060710.

Sequence in context: A049782 A091666 A084290 * A225520 A132857 A152782

Adjacent sequences:  A062008 A062009 A062010 * A062012 A062013 A062014

KEYWORD

nonn,easy

AUTHOR

Ahmed Fares (ahmedfares(AT)my-deja.com), Jul 12 2001

EXTENSIONS

More terms from Vladeta Jovovic, Jul 14 2001

Edited by N. J. A. Sloane, Sep 20 2018, replacing old definition (which was of course correct) with a simple formula.

STATUS

approved

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Last modified November 20 23:02 EST 2018. Contains 317427 sequences. (Running on oeis4.)