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

1, 2, 6, 16, 38, 138, 182, 209, 309, 341, 612, 1030, 1389, 2513, 2584, 2628, 2650, 2750, 3031, 3207, 3290, 3593, 3742, 3874, 3962, 4121, 4155, 4998, 5384, 5797, 6552, 6723, 6833, 7461, 7979, 8453, 8541, 8844, 8949, 9015, 9164, 9577, 10547, 10569, 11197, 11346

Offset: 1

Author

Enrique Navarrete, Apr 27 2017

Keywords

• nonn

Comments

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.

Links

• Robert Israel, Table of n, a(n) for n = 1..1570

Crossrefs

Cf. A068791.

Programs

Maple

N:= 40000: # for terms <= N/3
V:= Vector(N):
U:= [1,2]:
V[3]:= 1:
for i from 3 do
  found:= false;
  for j from U[i-1]+1 to N do
    if V[j]=1 then found:= true; break fi
  od;
  if not found then break fi;
  R:= select(`<=`,j+~U,N):
  V[R]:= 1 +~ V[R];
  U:= [op(U),j];
od:
U:= convert(U,set):
sort(convert(U intersect map(`*`,U,1/3),list)); # Robert Israel, Aug 31 2020