login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A227730 Doubly 2-densely divisible numbers. 2
1, 2, 4, 8, 12, 16, 24, 32, 36, 48, 60, 64, 72, 80, 96, 108, 112, 120, 128, 144, 160, 168, 180, 192, 216, 224, 240, 252, 256, 288, 300, 320, 324, 336, 360, 384, 396, 400, 420, 432, 448, 480, 504, 512, 528, 540, 560, 576, 600, 640, 648, 660, 672, 704, 720, 756, 768, 784 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that for each d | n, there is some 2-densely divisible (A174973) number d' such that d/2 < 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, 2), divisors(n))); if(!#d||2*d[#d]<n, return(0)); for(i=2, #d, if(d[i]>2*d[i-1], return(0))); 1

CROSSREFS

Cf. A227731, A174973.

Sequence in context: A047836 A325762 A231958 * A246692 A181824 A326677

Adjacent sequences:  A227727 A227728 A227729 * A227731 A227732 A227733

KEYWORD

nonn

AUTHOR

Charles R Greathouse IV, Jul 23 2013

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 29 15:16 EDT 2020. Contains 333107 sequences. (Running on oeis4.)