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
KEYWORD
nonn
AUTHOR
Enrique Navarrete, Apr 27 2017
STATUS
approved