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

1,2

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

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

Crossrefs

Cf. A068791.

Sequence in context: A265107 A217631 A046209 * A273348 A198951 A078774

Adjacent sequences: A285882 A285883 A285884 * A285886 A285887 A285888

Keyword

nonn

Author

Enrique Navarrete, Apr 27 2017

Status

approved