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A basic multiset identity says:

$$A+B = (A \cap B) + (A \cup B)$$

Allowing ourselves to use negative multiplicities and rearranging:

$$A-(A \cap B) = (A \cup B)-B$$

But since $A \supseteq (A \cap B),$ hence the LHS denotes a genuine (non-negative) multiset. Equivalently, since $(A \cup B) \supseteq B$, hence the RHS denotes a multiset. Of course, they're equal, so we've really just verified the same thing twice!

Anyway define that $A \mathop{-\!-} B$ equals $A - (A \cap B)$, or else $(A \cup B)-B$ if you prefer.

Question. Has anyone ever suggested a name or notation for the operation $\mathop{-\!-}$ on multisets?

This shows up naturally in the following way. Suppose we're living in a circle of $n$ beads. We begin taking steps of size $m$. How many steps must we take before we return to the starting point? Easy! Write $[n]$ for the multiset of prime factors of $n$ and $[m]$ for the multiset of prime factors of $n$. Write $\Pi(M)$ for the product of the elements of any multiset $M$ whose elements are numbers. Then the number of steps before we return to our starting point is $$\Pi([n]\mathop{-\!-}\,[m]).$$

(I haven't proved this, so its potentially completely wrong...)

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  • $\begingroup$ Isn't $A \setminus B$ what you're looking for? $\endgroup$ – Lord_Farin Aug 20 '15 at 18:57
  • $\begingroup$ @Lord_Farin, yep! Feel free to put that as an answer. $\endgroup$ – goblin Aug 20 '15 at 18:59
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In naive set theory, we usually have the operation $$A \setminus B := \{x: x \in A, x \notin B\}$$ For ordinary sets, we have: $$A \setminus B = A - (A \cap B) = (A \cup B) - B$$ although usually the latter two are also written using $\setminus$.

Therefore, it would seem reasonable to retain this definition for multisets. Indeed, this definition also preserves the identities with $\setminus$ for multisets: $$A \setminus B = A \setminus (A \cap B) = (A \cup B) \setminus B$$

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