OFFSET
1,2
EXAMPLE
There are no gaps with just one term, so we double to get a(2)=2. Still no gaps, so we double again to get a(3)=4. Now 3 has not appeared yet, so a(4)=3. The sequence is compact again, so we double to get a(5)=6.
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Greg Knowles, Jun 08 2011
STATUS
approved