cp's OEIS Frontend

A285885 Ulam numbers n such that 3*n is also an Ulam number.

%I A285885 #22 Aug 31 2020 19:18:05
%S A285885 1,2,6,16,38,138,182,209,309,341,612,1030,1389,2513,2584,2628,2650,
%T A285885 2750,3031,3207,3290,3593,3742,3874,3962,4121,4155,4998,5384,5797,
%U A285885 6552,6723,6833,7461,7979,8453,8541,8844,8949,9015,9164,9577,10547,10569,11197,11346
%N A285885 Ulam numbers n such that 3*n is also an Ulam number.
%C A285885 It appears that there are many more values in this sequence than in A068791, Ulam numbers n such that 2*n is also an Ulam number.
%H A285885 Robert Israel, <a href="/A285885/b285885.txt">Table of n, a(n) for n = 1..1570</a>
%p A285885 N:= 40000: # for terms <= N/3
%p A285885 V:= Vector(N):
%p A285885 U:= [1,2]:
%p A285885 V[3]:= 1:
%p A285885 for i from 3 do
%p A285885   found:= false;
%p A285885   for j from U[i-1]+1 to N do
%p A285885     if V[j]=1 then found:= true; break fi
%p A285885   od;
%p A285885   if not found then break fi;
%p A285885   R:= select(`<=`,j+~U,N):
%p A285885   V[R]:= 1 +~ V[R];
%p A285885   U:= [op(U),j];
%p A285885 od:
%p A285885 U:= convert(U,set):
%p A285885 sort(convert(U intersect map(`*`,U,1/3),list)); # _Robert Israel_, Aug 31 2020
%Y A285885 Cf. A068791.
%K A285885 nonn
%O A285885 1,2
%A A285885 _Enrique Navarrete_, Apr 27 2017