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 A070021 #10 May 14 2017 22:50:48
%S A070021 3,6,9,12,15,18,24,30,36,45,48,60,72,75,90,96,120,144,150,180,192,225,
%T A070021 240,288,300,360,375,384,450,480,576,600,720,750,768,900,960,1125,
%U A070021 1152,1200,1440,1500,1536,1800,1875,1920,2250,2304,2400,2880,3000,3072,3600
%N A070021 1/n has period 1 in base 10 (but not terminating).
%H A070021 Michael De Vlieger, <a href="/A070021/b070021.txt">Table of n, a(n) for n = 1..11778</a> (a(n) < 10^50)
%F A070021 Numbers of the form a*b*c where a is in {3, 9}, b is a power of 2 and c is a power of 5.
%e A070021 12 is in the sequence since 1/12=0.08333333333...; 11 is not since 1/11=0.0909090909...; 10 is not, since despite 1/10=0.099999999..., it is also true that 1/10=0.1 exactly.
%t A070021 mx = 5000; Select[ Union@ Flatten@ Table[2^t*5^f {3, 9}, {t, 0, Log[2, mx/3]}, {f, 0, Log[5, mx/(2^t*3)]}], # < mx &] (* _Robert G. Wilson v_, Apr 27 2017 *)
%Y A070021 Cf. A003592, A069105, A070022, A070023.
%K A070021 nonn,base,easy
%O A070021 1,1
%A A070021 _Henry Bottomley_, Apr 12 2002