This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A173394 #9 Feb 16 2025 08:33:12
%S A173394 1,3,9,11,17,33,51,73,99,101,137,153,187,219,257,303,353,411,449,561,
%T A173394 641,657,771,803,909,1059,1111,1233,1241,1347,1409,1507,1683,1717,
%U A173394 1923,2313,2329,2409,2827,3177,3333,3723,3883,4041,4227,4369,4521,4939
%N A173394 Positions of powers of 2 in A084680.
%C A173394 The n where the decimal expansion of 1/n has a period length which is a power of 2.
%D A173394 J. W. L. Glaisher, On circulating decimals, Proc. Camb. Phil. Soc., 3 (1878), 185-206.
%D A173394 D. H. Lehmer, Guide to Tables in the Theory of Numbers. Bulletin No. 105, National Research Council, Washington, DC, 1941, pp. 7-12.
%H A173394 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/DecimalExpansion.html">Decimal Expansion</a>.
%F A173394 {n: A084680(n) in A000079}.
%e A173394 Period of 1/3 = 1. Period of 1/9 = 2. Period of 1/73 = 8. Period of 1/353 = 32.
%p A173394 A084680 := proc(n) if gcd(n,10) <> 1 then 0 ; elif n = 1 then 1 ; else numtheory[order](10,n) ; end if; end proc:
%p A173394 isA000079 := proc(n) if n = 1 then true; elif n >=1 then numtheory[factorset](n) = {2} ; else false; end if; end proc:
%p A173394 for n from 1 to 5000 do if isA000079(A084680(n)) then printf("%d,",n) ; end if; end do: # _R. J. Mathar_, Mar 10 2010
%Y A173394 Cf. A002329.
%K A173394 nonn
%O A173394 1,2
%A A173394 _Michel Lagneau_, Feb 17 2010
%E A173394 Missing numbers -those where A084680(n)>1000- inserted - _R. J. Mathar_, Mar 10 2010