Thanks for the hint to go looking some more. I found that Johannes Riebel has proven that BB(748) is undecidable. So for even small k there may not be deciders for them.
Thanks for the hint to go looking some more. I found that Johannes Riebel has proven that BB(748) is undecidable. So for even small k there may not be deciders for them.
The suspicion is that this happens maybe as early as BB(15). We just can't prove that whereas we can prove BB(745) is not decidable, and, of course, we've decided BB(5) as we see here.