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A202160 a(n) = smallest k having at least five prime divisors d such that (d + n) | (k + n). 1
588455, 179998, 460317, 6265805, 1236235, 287274, 949025, 1436932, 794871, 2013650, 3797365, 1169688, 3739827, 1587586, 6872565, 7706270, 1529983, 7351242, 2528045, 5247970, 487179, 10920965, 1316497, 121894476, 1404455, 5814874, 12223653, 2260412, 8022531 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The sequence of numbers k composite and squarefree, prime p | k ==> p+n | k+n is given by A029591 (least quasi-Carmichael number of order -n).

If k is squarefree, for n = 1, we obtain Lucas-Carmichael numbers: A006972.

In this sequence, the majority of terms are not squarefree.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..125

EXAMPLE

a(3) = 460317 because the prime divisors of 460317 are 3, 11, 13, 29, 37  =>

(3 + 3) | (460317 + 3) = 460320 = 6*76720;

(11 + 3) | 460320 = 14*32880;

(13 + 3) | 460320 = 16*28770;

(29+3)  |  460320 = 32*14385;

(37+3) | 460320 = 40*11508.

MAPLE

with(numtheory):for n from 1 to 23 do:i:=0:for k from 1 to 10^8 while(i=0) do:x:=factorset(k):n1:=nops(x):y:=k+n: j:=0:for m from 1 to n1 do:if  n1>=2 and irem(y, x[m]+n)=0 then j:=j+1:else fi:od:if j>4 then i:=1: printf ( "%d %d \n", n, k):else fi:od:od:

MATHEMATICA

numd[n_, k_] := Module[{p=FactorInteger[k][[;; , 1]], c=0}, Do[If[Divisible[n+k, n+p[[i]]], c++], {i, 1, Length[p]}]; c]; a[n_]:=Module[{k=1}, While[numd[n, k] <= 4, k++]; k]; Array[a, 30] (* Amiram Eldar, Sep 09 2019 *)

CROSSREFS

Cf. A006972, A029591, A202157, A202158, A202159.

Sequence in context: A100386 A205214 A206222 * A216927 A090871 A237493

Adjacent sequences:  A202157 A202158 A202159 * A202161 A202162 A202163

KEYWORD

nonn

AUTHOR

Michel Lagneau, Dec 13 2011

STATUS

approved

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Last modified February 27 11:24 EST 2021. Contains 341656 sequences. (Running on oeis4.)