A227731 - OEIS

A227731

Doubly 3-densely divisible numbers.

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, 81, 84, 90, 96, 100, 108, 112, 120, 126, 128, 135, 140, 144, 150, 160, 162, 168, 176, 180, 189, 192, 200, 210, 216, 220, 224, 225, 240, 243, 252, 256, 264, 270, 280, 288, 300, 315

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OFFSET

1,2

COMMENTS

Numbers n such that for each d | n, there is some 3-densely divisible (A196149) number d' such that d/3 < d' <= d.

LINKS

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

Terence Tao, The distribution of primes in doubly densely divisible moduli

PROG

(PARI) dense(n, k)=my(d=divisors(n)); for(i=2, #d, if(d[i]>k*d[i-1], return(0))); 1

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

CROSSREFS