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A059267 Numbers n with 2 divisors d1 and d2 having difference 2: d2 - d1 = 2; equivalently, numbers that are 0 (mod 4) or have a divisor d of the form d = m^2 - 1. 5
3, 4, 6, 8, 9, 12, 15, 16, 18, 20, 21, 24, 27, 28, 30, 32, 33, 35, 36, 39, 40, 42, 44, 45, 48, 51, 52, 54, 56, 57, 60, 63, 64, 66, 68, 69, 70, 72, 75, 76, 78, 80, 81, 84, 87, 88, 90, 92, 93, 96, 99, 100, 102, 104, 105, 108, 111, 112, 114, 116, 117, 120, 123, 124, 126, 128 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A099475(a(n)) > 0: complement of A099477; A008586, A008585 and A037074 are subsequences - Reinhard Zumkeller, Oct 18 2004

These numbers have an asymptotic density of ~ 0.522. This corresponds to all numbers which are multiples of 4 (25%), or of 3 (having 1 & 3 as divisors: + (1-1/4)*1/3 = 1/4), or of 5*7, or of 11*13, etc. (Generally, multiples of lcm(k,k+2), but multiples of 3 and 4 are already taken into account in the 50% covered by the first 2 terms.) - M. F. Hasler, Jun 02 2012

LINKS

M. F. Hasler, Table of n, a(n) for n = 1..3131.

EXAMPLE

a(18) = 35 because 5 and 7 divide 35 and 7 - 5 = 2

MAPLE

with(numtheory): for n from 1 to 1000 do flag := 1: if n mod 4 = 0 then printf(`%d, `, n):flag := 0 fi: for m from 2 to ceil(sqrt(n)) do if n mod (m^2-1) = 0 and flag=1 then printf(`%d, `, n); break fi: od: od:

MATHEMATICA

d1d2Q[n_]:=Mod[n, 4]==0||AnyTrue[Sqrt[#+1]&/@Divisors[n], IntegerQ]; Select[ Range[ 200], d1d2Q] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 31 2020 *)

PROG

(PARI) isA059267(n)={ n%4==0 || fordiv( n, d, issquare(d+1) && return(1))} \\ M. F. Hasler, Aug 29 2008

(PARI) is_A059267(n) = fordiv( n, d, n%(d+2)||return(1)) \\ M. F. Hasler, Jun 02 2012

CROSSREFS

Sequence in context: A192519 A036446 A284469 * A049433 A250984 A135251

Adjacent sequences:  A059264 A059265 A059266 * A059268 A059269 A059270

KEYWORD

nonn

AUTHOR

Avi Peretz (njk(AT)netvision.net.il), Jan 23 2001

EXTENSIONS

More terms from James A. Sellers, Jan 24 2001

Removed comments linking to A143714, which seem wrong, as observed by Ignat Soroko, M. F. Hasler, Jun 02 2012

STATUS

approved

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Last modified November 30 14:29 EST 2020. Contains 338802 sequences. (Running on oeis4.)