A285885 - OEIS

%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