Slava's Monoid Zoo

2 comments

• voidUpdate 3 hours ago
  > "Can you see a way to transform a string of 8 apples "" into a string of 10 apples ""?"

  Am I missing something? The only rules we have are BAB -> AAA and BBB -> BB, and we start with only A, no B, so neither of those rules can be used, so the answer is no?

  EDIT: Ah, looks like you cant put emoji in HN comments. Imagine there's apples in there

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  • Phemist 3 hours ago
    The relations are bi-directional. So you can change AAA -> BAB and BB -> BBB as well.
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    • dhosek 2 hours ago
      Yeah, that was the secret sauce that left me a bit confused
    • voidUpdate 3 hours ago
      Oh, I see. That makes sense
• entaloneralie 3 hours ago
  I'm too scared to leave the comfy world of commutative monoids.
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  • Sesse__ 1 hour ago
    Is the word problem easier if the monoids are commutative? (Or even trivial? I haven't thought deeply about it.)
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    • hyperpape 1 hour ago
      I haven't previously thought about this, but I think words over a commutative monoid are equivalent to a vector of non-negative integers, at which point you have vector addition systems, and I believe those are decidable, though still computationally incredibly hard: https://www.quantamagazine.org/an-easy-sounding-problem-yiel....
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      • Sesse__ 1 hour ago
        Thanks, that's an interesting tidbit!

        (The whole thing made me think about applications to SQL query optimizers, although I'm not sure if it's practically useful for anything.)