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