A336323 - OEIS

%I #18 Aug 10 2020 00:47:55

%S 6,10,12,14,18,21,24,25,26,28,32,33,34,36,39,42,45,48,50,52,60,68,70,

%T 72,74,76,77,78,84,86,87,88,90,95,98,100,102,104,105,106,110,111,112,

%U 124,128,130,132,135,138,140,145,147,150,158,160,162,165,168,170

%N Numbers composite(n) such that gcd(n,composite(n)) > 1.

%H Robert Israel, <a href="/A336323/b336323.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A002808(A334880(n)). - _Michel Marcus_, Jul 19 2020

%p compn:= 0: R:= NULL: count:= 0:

%p for n from 4 while count < 100 do

%p   if not isprime(n) then

%p     compn:= compn+1;

%p     if igcd(compn,n) > 1 then

%p       R:= R, n; count:= count+1;

%p     fi

%p   fi

%p od:

%p R; # _Robert Israel_, Aug 09 2020

%t c = Select[Range[2, 250], ! PrimeQ[#] &]; (*  A002808 *)

%t u = Select[Range[Length[c]], GCD[c[[#]], #] > 1 &] ; (* A334880 *)

%t Table[c[[u[[n]]]], {n, 1, Length[u]}] (* A336323 *)

%o (PARI) lista(nn) = {my(n=0, list=List()); forcomposite (c=1, nn, n++; if (gcd(n, c) > 1, listput(list, c))); Vec(list);} \\ _Michel Marcus_, Jul 19 2020

%Y Cf. A002808, A334880.

%K nonn,easy

%O 1,1

%A _Clark Kimberling_, Jul 18 2020