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 A227731 #9 Nov 30 2014 13:25:22 %S A227731 1,2,3,4,6,8,9,12,16,18,20,24,27,30,32,36,40,45,48,54,60,63,64,72,80, %T A227731 81,84,90,96,100,108,112,120,126,128,135,140,144,150,160,162,168,176, %U A227731 180,189,192,200,210,216,220,224,225,240,243,252,256,264,270,280,288,300,315 %N A227731 Doubly 3-densely divisible numbers. %C A227731 Numbers n such that for each d | n, there is some 3-densely divisible (A196149) number d' such that d/3 < d' <= d. %H A227731 Charles R Greathouse IV, <a href="/A227731/b227731.txt">Table of n, a(n) for n = 1..10000</a> %H A227731 Terence Tao, <a href="http://terrytao.wordpress.com/2013/07/07/the-distribution-of-primes-in-doubly-densely-divisible-moduli/">The distribution of primes in doubly densely divisible moduli</a> %o A227731 (PARI) dense(n,k)=my(d=divisors(n)); for(i=2, #d, if(d[i]>k*d[i-1], return(0))); 1 %o A227731 is(n)=my(d=select(k->dense(k,3),divisors(n))); if(!#d||3*d[#d]<n,return(0)); for(i=2, #d, if(d[i]>3*d[i-1], return(0))); 1 %Y A227731 Cf. A227730, A196149. %K A227731 nonn %O A227731 1,2 %A A227731 _Charles R Greathouse IV_, Jul 23 2013