A288162 Numbers whose prime factors are 2 and 13.

26, 52, 104, 208, 338, 416, 676, 832, 1352, 1664, 2704, 3328, 4394, 5408, 6656, 8788, 10816, 13312, 17576, 21632, 26624, 35152, 43264, 53248, 57122, 70304, 86528, 106496, 114244, 140608, 173056, 212992, 228488, 281216, 346112, 425984, 456976, 562432, 692224, 742586, 851968, 913952

Offset

1,1

Comments

Numbers k such that phi(k)/k = 6/13.

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Formula

a(n) = 26 * A107326(n). - David A. Corneth, Jun 06 2017

Sum_{n>=1} 1/a(n) = 1/12. - Amiram Eldar, Dec 22 2020

Mathematica

Select[Range[920000], FactorInteger[#][[All, 1]]=={2, 13}&] (* Harvey P. Dale, Jun 18 2021 *)

Prog

(PARI) is(n) = factor(n)[, 1]~==[2, 13] \\ Felix Fröhlich, Jun 06 2017
(PARI) list(lim)=my(v=List(), t); for(n=1, logint(lim\2, 13), t=13^n; while((t<<=1)<=lim, listput(v, t))); Set(v) \\ Charles R Greathouse IV, Jun 11 2017
(Magma) [n:n in [1..100000] | Set(PrimeDivisors(n)) eq {2, 13}];  // Marius A. Burtea, May 10 2019

Crossrefs

Cf. A033845, A033846, A033847, A033848, A033849, A033850, A033851, A086780, A107326, A143207.

Sequence in context: A122999 A040650 A252994 * A352346 A214476 A121738

Adjacent sequences: A288159 A288160 A288161 * A288163 A288164 A288165

Keyword

nonn

Author

Bernard Schott, Jun 06 2017

Status

approved