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A227731 Doubly 3-densely divisible numbers. 2
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 (list; graph; refs; listen; history; text; internal format)
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

Cf. A227730, A196149.

Sequence in context: A010434 A074230 A064438 * A067947 A279542 A053640

Adjacent sequences:  A227728 A227729 A227730 * A227732 A227733 A227734

KEYWORD

nonn

AUTHOR

Charles R Greathouse IV, Jul 23 2013

STATUS

approved

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Last modified October 19 16:17 EDT 2019. Contains 328223 sequences. (Running on oeis4.)