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Numbers whose number of divisors is divisible by 7.
2

%I #12 May 05 2023 09:38:57

%S 64,192,320,448,576,704,729,832,960,1088,1216,1344,1458,1472,1600,

%T 1728,1856,1984,2112,2240,2368,2496,2624,2752,2880,2916,3008,3136,

%U 3264,3392,3520,3645,3648,3776,3904,4032,4160,4288,4416,4544,4672,4800,4928,5056,5103

%N Numbers whose number of divisors is divisible by 7.

%C The asymptotic density of this sequence is 1 - zeta(7)/zeta(6) = 0.0088404638... (Sathe, 1945).

%H Amiram Eldar, <a href="/A336596/b336596.txt">Table of n, a(n) for n = 1..10000</a>

%H Eckford Cohen, <a href="https://eudml.org/doc/140760">Arithmetical Notes, XIII. A Sequal to Note IV</a>, Elemente der Mathematik, Vol. 18 (1963), pp. 8-11.

%H S. S. Pillai, <a href="https://doi.org/10.18311/jims/1942/17182">On a congruence property of the divisor function</a>, J. Indian Math. Soc. (N. S.), Vol. 6, (1942), pp. 118-119.

%H L. G. Sathe, <a href="https://www.jstor.org/stable/2371953">On a congruence property of the divisor function</a>, American Journal of Mathematics, Vol. 67, No. 3 (1945), pp. 397-406.

%F A030516 UNION A030632 UNION A137484 UNION A137491 UNION A175745 UNION A175750 UNION ... - _R. J. Mathar_, May 05 2023

%e 64 is a term since A000005(64) = 7 is divisible by 7.

%p q:= n-> is(irem(numtheory[tau](n), 7)=0):

%p select(q, [$1..5500])[]; # _Alois P. Heinz_, Jul 26 2020

%t Select[Range[5000], Divisible[DivisorSigma[0, #], 7] &]

%Y Cf. A000005, A008589, A059269, A336595.

%Y Cf. A030516, A113851 and A138031 are subsequences.

%K nonn

%O 1,1

%A _Amiram Eldar_, Jul 26 2020