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%I #9 Nov 30 2014 13:25:22
%S 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 81,84,90,96,100,108,112,120,126,128,135,140,144,150,160,162,168,176,
%U 180,189,192,200,210,216,220,224,225,240,243,252,256,264,270,280,288,300,315
%N Doubly 3-densely divisible numbers.
%C Numbers n such that for each d | n, there is some 3-densely divisible (A196149) number d' such that d/3 < d' <= d.
%H Charles R Greathouse IV, <a href="/A227731/b227731.txt">Table of n, a(n) for n = 1..10000</a>
%H 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 (PARI) dense(n,k)=my(d=divisors(n)); for(i=2, #d, if(d[i]>k*d[i-1], return(0))); 1
%o 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 Cf. A227730, A196149.
%K nonn
%O 1,2
%A _Charles R Greathouse IV_, Jul 23 2013
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