There's actually a kicker to all that.
The barman asks whether the mathematicians are aware of their limits.
There is, of course, a pun on the idea of having a drinking limit for sobriety, but in the mathematical realm, it also alludes to the aspect of infinitude.
Whereas theoretical geometric progressions approaching zero can, by definition, never reach zero (the theoretical hyperbolic line and its axis), we are dealing here with matter.
So the unfortunate barman will ultimately be confronted with his inability to split an atom.
Hence the limit of which he reminds them.
Mind you,m the bar is getting crowded by then...