A226011 - OEIS

%I #24 Jan 15 2022 21:40:25

%S 1,2,4,18,72,130,280,495,840,900,930,1216,1880,2646,3182,3762,17437,

%T 21600,29400,30654,35207,45024,58230,266133,1051980,1204164,1204320,

%U 2193408,2494520,2549745,3272178,3671570,3843378,5166708,5398785,8709805,8922270,8922480

%N Numbers k such that nonprime(k) divides k^2.

%e The 4th nonprime is 8, and 8 divides 4^2 = 16, so 8 is in the sequence.

%p for n from 1 do

%p     if n^2 mod A018252(n) = 0 then

%p         print(n) ;

%p     end if;

%p end do: # _R. J. Mathar_, Jun 04 2013

%t seq = {}; np = n = 0; p = 2; While[n < 10^5, If[++np == p, p = NextPrime@p, n++; If[Mod[n^2, np] == 0, Print@{n, np, n^2/np}; AppendTo[seq, n]]]]; seq (* _Giovanni Resta_, May 27 2013 *)

%t Module[{nn=10^7,np,len},np=Complement[Range[nn],Prime[ Range[ PrimePi[ nn]]]];len=Length[np];Transpose[Select[ Thread[ {Range[len],np}], Divisible[ First[#]^2, Last[#]]&]][[1]]] (* _Harvey P. Dale_, Sep 10 2014 *)

%Y Cf. A018252.

%K nonn,less

%O 1,2

%A _Gerasimov Sergey_, May 27 2013

%E a(5)-a(38) from _Giovanni Resta_, May 27 2013