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Non-commutative rings and algebras, non-associative algebras, universal algebra and lattice theory, linear algebra, semigroups. For questions specific to commutative algebra (that is, rings that are assumed both associative and commutative), rather use the tag ac.commutative-algebra.

3 votes
Accepted

A complete lattice of functions

Once one adds infinity, it is easy to check that $(P', \leq)$ is indeed a complete lattice. For any subset $A$ of $P'$, it is easy to see that $\vee A$ is just the pointwise supremum of members of $A …
Tony Huynh's user avatar
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32 votes
Accepted

Function of $(x_1,x_2,x_3,x_4)$ that factors in two ways as $\phi_1 (x_1 ,x_2 )\phi_2(x_3 ,x...

Here is a fairly straightforward proof which also proves various generalizations of your problem. Choose $c,d$ such that $\phi_2(c,d) \neq 0$. If no such $c,d$ exist, then $f$ is identically $0$ and …
Tony Huynh's user avatar
  • 32.1k
8 votes
1 answer
238 views

Functions over monoids which factor in two different ways

This is a follow-up question to this MO question, which was asked by Richard Stanley in a comment to my answer there. Let $S$ be a commutative monoid and $f(x_1, \dots, x_n)$ be a function from $S^n$ …
Tony Huynh's user avatar
  • 32.1k
6 votes

Proving that a poset is a lattice

If you have a handle on the join operator, then you can try to show that your poset is a complete join semi-lattice. This shows that your poset is also a complete meet semi-lattice, and hence a compl …
Tony Huynh's user avatar
  • 32.1k