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Let $p$ be an element of a commutative ring with unity. The following definition is natural:

$p$ is minimal under division if its only divisors (up to equivalence) are $1$ and itself. That is, for all $a$, if $(p) \subseteq (a)$ then $(a) = (1)$ or $(a) = (p)$.

Although closely related, in a ring with zero divisors, this differs from the usual notions of irreducible and prime:

  • In the ring $\mathbb{Z}[x, y] / (xy)$, the ideal $(x)$ is prime, but not minimal under division. In particular, $x$ reduces as $x = x (1 - y)$, and $(1 - y)$ is an ideal strictly between $(x)$ and $(1)$.
  • In the ring $\mathbb{Z} / 6\mathbb{Z}$, the ideal $(2) = (4)$ is minimal under division, but not irreducible since $4 = 4 \cdot 4$. (It is also prime.)

I'm looking for references about this concept. What is such an element usually called?

Notes and related questions

  • If $(p)$ is a maximal ideal then $p$ is minimal under division, but not necessarily vice versa.

  • If $p$ is irreducible then $p$ is minimal under division, but not necessarily vice versa.

  • This question shows that "minimal under division" and "irreducible" coincide in a principal ideal domain (or, more generally, for non-zero-divisors in a principal ideal ring).

  • This answer points out that factorization theory is much more complicated in non-domains, and there are several (inequivalent) notions of irreducibility.

1 Answers1

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Answering my own question. I decided to take a look at Anderson and Valdes-Leon's paper (1996). They do consider this notion, and they call it m-irreducible (p. 448):

A nonunit $a \in R$ is m-irreducible if $(a)$ is a maximal element in the set of proper ideals of $R$.

This notion is related to three other notions of irreducibility, none of them equivalent (see also the discussion at this MO answer). Here is a diagram of all the relevant notions ordered by inclusion (most of these I know are strict, but I may be missing some inclusions):

maximal     very strongly irreducible
|      \   /
|       m-irreducible
prime   |
|       strongly irreducible
|       |
weakly irreducible

Maximal and prime are standard ($p$ is prime if $(p)$ is a prime ideal and $p$ is maximal if $(p)$ is a maximal ideal). The rest of the categories above can be restated as follows, in decreasing order of strength. I use weakly irreducible for what they call irreducible to avoid confusion with the usual definition.

  • $p$ is very strongly irreducible if it is a nonunit and whenever $p = ab$, either $a$ or $b$ is a unit. (Note: this is the usual definition of irreducibility that I am familiar with.)

  • $p$ is m-irreducible if it is a nonunit and whenever $p = ab$, $(a) = (1)$ or $(a) = (p)$. (Note: this is the definition of minimal under division in the OP.)

  • $p$ is strongly irreducible if it is a nonunit and whenever $p = ab$, there exists a unit $u$ such that $p = ua$ or $p = ub$.

  • $p$ is weakly irreducible if it is a nonunit and whenever $p = ab$, we have $(a) = (p)$ or $(b) = (p)$. (Note: this is similar to, but weaker than $p$ being prime.)

The following is adapted from Anderson and Valdes-Leon (Theorem 2.13, p. 450):

Theorem 2.13. Let $a \in R$. Then $a$ very strongly irreducible $\implies$ $a$ is $m$-reducible $\implies$ $a$ is strongly irreducible $\implies$ $a$ is weakly irreducible.

Moreover, none of these implications can be reversed.

Proof. We show each of the three inclusions in turn (the second inclusion is the most interesting).

  • First, $p$ is very strongly irreducible. For any decomposition $p = ab$, either $a$ is a unit or $b$ is a unit. In the former case, $(a) = (1)$. In the latter case, $(b) = (1)$, so $(a) = (a)(1) = (a)(b) = (p)$. Thus, $p$ is m-irreducible.

  • Next, suppose $p$ is m-irreducible. For any decomposition $p = ab$, if either $(a) = (1)$ or $(b) = (1)$ then we are done as $p$ is equal to one of $a$ or $b$ times a unit. Thus it remains to consider $(a) = (b) = (p)$. In this case, we can write $a = pa'$ and $b = pb'$, so $p = ab = p^2 a' b'$. Letting $r = a' b'$, we have $p = p^2r$ so $pr = (pr)^2$, i.e. $pr$ is idempotent. Also, $(pr) = (p)$. Let $e = pr$ be the idempotent.

    Now we claim that, for any $x$, if $(x) = (e)$ then $x = eu$ for some unit u. For this, consider the quantity $$ u = ex + (1-e). $$ Multiplying by $e$ we have $eu = ex + 0 = ex = x$ since $e$ divides $x$. But this is also a unit: since $x$ divides $e$ we have $e = xy$, then letting $v = ey + (1-e)$ we get $$ uv = exy + (1-e)^2 = e + 1-e = 1, $$ hence $x = eu$ and $u$ is a unit.

    Applying this result both $x = a$ and $x = p$, we get that $a = eu_1$ and $p = eu_2$, hence $a$ and $p$ are equivalent up to multiplication by a unit. So $p$ is strongly irreducible.

  • Finally, suppose $p$ is strongly irreducible. For any decomposition $p = ab$, suppose WLOG $p = ua$. Then $(p) = (a)$. Thus, $p$ is weakly irreducible.

Notes

  • Anderson and Valdes-Leon also say in Theorem 2.13 that $p$ is strongly irreducible implies that $a$ is both weakly irreducible and prime. But this seems like a mistake in the text; they don't prove it, and it's false for integral domains. That is, it is well-known that there are examples of irreducible (hence very strongly irreducible) elements that are not prime; see this question. The rest of the theorem seems OK.

  • Anderson and Valdes-Leon derive weakly, strongly, and very strongly irreducible definitions from corresponding relations $\sim$, $\approx$, $\approxeq$ (associates, strong associates, and very strong associates) on $R$. Above I've given direct definitions (using some of the equivalent statements) without using $\sim$, $\approx$, and $\approxeq$.

  • My definition of "very strongly irreducible" differs from Anderson and Valdes-Leon in case $p = 0$. In that case, they decide to special case $0 \approxeq 0$, and it follows that $0$ is very strongly irreducible precisely when $R$ is a domain. But this later becomes awkward, as it requires them to exclude $a \ne 0$ in Theorem 2.13. With the above definitions the theorem holds for $a = 0$: $0$ is never very strongly irreducible; it's $m$-reducible exactly when $R$ is a field; and it's strongly and weakly irreducible precisely when $R$ is a domain.