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Jeremy Rickard
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The example I originally posted doesn't work.

Here are a couple of This is way too addictive, so I'm going to try to quit, and I'll just leave my thoughts here in case they're useful for other addicts. This is based on the ideas in the previous non-answer that might lead to an example that's not a Boolean ringI posted last year, so I'm just editing that answer. I'll denote

Denote the number of ideals of a ring $A$$R$ by $n(A)$$n(R)$, and define the Alexeq Quotient $q(R)$ to be $\frac{n(R)}{\vert R\vert}$, so the question asks about rings $R$ with $q(R)=1$.

$q(R)$ is multiplicative on direct products (1i.e., $q(R\times S)=q(R)q(S)$) Suppose you, so one approach to finding rings with $q(R)=1$ is to look for examples with fairly simple fractions as $q(R)$ and take products.

For example, $q(\mathbb{F}_p)=\frac{2}{p}$, and $q(\mathbb{F}_4)=\frac{1}{2}$, so if we could find a ring with $A$ of order$q(R)=\frac{m}{2^k}$, where $2^k$ such that the number of$m$ has at least $k$ prime factors (counted with multiplicity) of $n(A)$ is at least $k$. Suppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ hasthen by taking a suitable direct product with fields we could achieve $2^{2l-k}p_1\dots p_l$ elements and ideals$q(R)=1$.

It seems quiteIt's not hard to produce examples. I triedfind $A=\mathbb{F}_2\oplus V$$\mathbb{F}_2$-algebras with $q(R)>1$. For example, take $R$ to be the $d$-dimensional algebra $\mathbb{F}_2\oplus V$ where $V$ is ana $\mathbb{F}_2$$(d-1)$-vector space and adimensional square zero-zero ideal infor $A$$d>3$.

However, so the proper ideals of $A$ are$n(R)$ is bounded by the number of subspaces of $V$$R$, but towhich behaves roughly as a constant times $2^{d^2/4}$ for large values of $d=\dim(R)$. But the limitnumber of my calculationsprime factors of a "typical" large number $n(A)$$n$ is roughly $\log\log n$, giving roughly $\log d$, so unless we can carefully design $R$ so that $n(R)$ has far too fewmany prime factors, and very rough heuristics suggest that this will continuewe probably need to be lucky to find an $\mathbb{F}_2$-algebra where $n(R)$ has as many prime factors as $\vert R\vert$.

(2) Here's a way of producing a ring $B$ It's possible to find rings where the number of$n(R)$ has more prime factors ofthan $n(B)$$\vert R\vert$. For example, $R=\mathbb{F}_q[x,y]/(x^2,y^2)$ has $n(R)=q+5$ and so $q(R)=\frac{q+5}{q^4}$. So if $q$ is a reasonably large compared to the numberprime of the form $2^k-5$ (e.g., $q=59$) then $n(R)$ has $2^k$ prime factors of, but $|B|$$\vert R\vert$ has only $4$.

Let $p$ be aUnfortunately, I don't know any way to do this without introducing large prime such that $p+5=2^t$ is a powerfactors in the denominator of $2$$q(R)$, andwhich are hard to get rid of: it's difficult to design $t>4$. Then$S$ so that $B=\mathbb{F}_p[x,y]/(x^2,y^2)$ has$n(S)$ is divisible by a particular prime, but $|B|=p^4$ and$n(S)$ doesn't have many fewer prime factors than $n(B)=2^t$$\vert S\vert$.

If youA weakening of the original question, that I don't think I've found an answer to, is:

Question: Is there a finite commutative ring as in$R$ (1apart from finite boolean rings) but with, where $l$ slightly smaller$n(R)\geq \vert R\vert$ and $n(R)$ has at least as many prime factors than $k$$\vert R\vert$?

I found one example that might be useful. If $R=\mathbb{F}_2[x]/(x^5)\oplus \mathbb{F}_2^4$, and where $p^4$ divides $n(A)$$\mathbb{F}_2^4$ is a square zero ideal, then considering $A\times B$$n(R)=1296=2^43^4$, and continuing as inso (1) might work$q(R)=\frac{3^4}{2^5}$. This means that if we could find a $d$-dimensional $\mathbb{F}_3$-algebra $S$ where $n(S)$ has at least $\frac{5d}{4}$ prime factors then we could construct $T$ with $q(T)=1$.

The example I originally posted doesn't work.

Here are a couple of ideas that might lead to an example that's not a Boolean ring. I'll denote the number of ideals of a ring $A$ by $n(A)$.

(1) Suppose you could find a ring $A$ of order $2^k$ such that the number of prime factors (counted with multiplicity) of $n(A)$ is at least $k$. Suppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ has $2^{2l-k}p_1\dots p_l$ elements and ideals.

It seems quite hard to produce examples. I tried $A=\mathbb{F}_2\oplus V$, where $V$ is an $\mathbb{F}_2$-vector space and a square zero ideal in $A$, so the proper ideals of $A$ are the subspaces of $V$, but to the limit of my calculations $n(A)$ has far too few prime factors, and very rough heuristics suggest that this will continue.

(2) Here's a way of producing a ring $B$ where the number of prime factors of $n(B)$ is large compared to the number of prime factors of $|B|$.

Let $p$ be a prime such that $p+5=2^t$ is a power of $2$, and $t>4$. Then $B=\mathbb{F}_p[x,y]/(x^2,y^2)$ has $|B|=p^4$ and $n(B)=2^t$.

If you found a ring as in (1) but with $l$ slightly smaller than $k$, and where $p^4$ divides $n(A)$, then considering $A\times B$ and continuing as in (1) might work.

This is way too addictive, so I'm going to try to quit, and I'll just leave my thoughts here in case they're useful for other addicts. This is based on the ideas in the previous non-answer that I posted last year, so I'm just editing that answer.

Denote the number of ideals of $R$ by $n(R)$, and define the Alexeq Quotient $q(R)$ to be $\frac{n(R)}{\vert R\vert}$, so the question asks about rings $R$ with $q(R)=1$.

$q(R)$ is multiplicative on direct products (i.e., $q(R\times S)=q(R)q(S)$), so one approach to finding rings with $q(R)=1$ is to look for examples with fairly simple fractions as $q(R)$ and take products.

For example, $q(\mathbb{F}_p)=\frac{2}{p}$, and $q(\mathbb{F}_4)=\frac{1}{2}$, so if we could find a ring with $q(R)=\frac{m}{2^k}$, where $m$ has at least $k$ prime factors (counted with multiplicity) then by taking a suitable direct product with fields we could achieve $q(R)=1$.

It's not hard to find $\mathbb{F}_2$-algebras with $q(R)>1$. For example, take $R$ to be the $d$-dimensional algebra $\mathbb{F}_2\oplus V$ where $V$ is a $(d-1)$-dimensional square-zero ideal for $d>3$.

However, $n(R)$ is bounded by the number of subspaces of $R$, which behaves roughly as a constant times $2^{d^2/4}$ for large values of $d=\dim(R)$. But the number of prime factors of a "typical" large number $n$ is roughly $\log\log n$, giving roughly $\log d$, so unless we can carefully design $R$ so that $n(R)$ has many prime factors, we probably need to be lucky to find an $\mathbb{F}_2$-algebra where $n(R)$ has as many prime factors as $\vert R\vert$.

It's possible to find rings where $n(R)$ has more prime factors than $\vert R\vert$. For example, $R=\mathbb{F}_q[x,y]/(x^2,y^2)$ has $n(R)=q+5$ and so $q(R)=\frac{q+5}{q^4}$. So if $q$ is a reasonably large prime of the form $2^k-5$ (e.g., $q=59$) then $n(R)$ has $2^k$ prime factors, but $\vert R\vert$ has only $4$.

Unfortunately, I don't know any way to do this without introducing large prime factors in the denominator of $q(R)$, which are hard to get rid of: it's difficult to design $S$ so that $n(S)$ is divisible by a particular prime, but $n(S)$ doesn't have many fewer prime factors than $\vert S\vert$.

A weakening of the original question, that I don't think I've found an answer to, is:

Question: Is there a finite commutative ring $R$ (apart from finite boolean rings), where $n(R)\geq \vert R\vert$ and $n(R)$ has at least as many prime factors than $\vert R\vert$?

I found one example that might be useful. If $R=\mathbb{F}_2[x]/(x^5)\oplus \mathbb{F}_2^4$, where $\mathbb{F}_2^4$ is a square zero ideal, then $n(R)=1296=2^43^4$, and so $q(R)=\frac{3^4}{2^5}$. This means that if we could find a $d$-dimensional $\mathbb{F}_3$-algebra $S$ where $n(S)$ has at least $\frac{5d}{4}$ prime factors then we could construct $T$ with $q(T)=1$.

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Jeremy Rickard
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The example I originally posted doesn't work.

Here's an ideaHere are a couple of ideas that might lead to an example that's not a Boolean ring. I'll denote the number of ideals of a ring $A$ by $n(A)$.

(1) Suppose you could find a ring $A$ of order $2^k$ with $n(A)$ ideals such that the number of prime factors (counted with multiplicity) of $n(A)$ is at least $k$. Suppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ has $2^{2l-k}p_1\dots p_l$ elements and ideals.

It seems quite hard to produce examples. I tried $A=\mathbb{F}_2\oplus V$, where $V$ is an $\mathbb{F}_2$-vector space and a square zero ideal in $A$, so the proper ideals of $A$ are the subspaces of $V$, but to the limit of my calculations $n(A)$ has far too few prime factors, and very rough heuristics suggest that this will continue.

(2) Here's a way of producing a ring $B$ where the number of prime factors of $n(B)$ is large compared to the number of prime factors of $|B|$.

Let $p$ be a prime such that $p+5=2^t$ is a power of $2$, and $t>4$. Then $B=\mathbb{F}_p[x,y]/(x^2,y^2)$ has $|B|=p^4$ and $n(B)=2^t$.

If you found a ring as in (1) but with $l$ slightly smaller than $k$, and where $p^4$ divides $n(A)$, then considering $A\times B$ and continuing as in (1) might work.

The example I originally posted doesn't work.

Here's an idea that might lead to an example that's not a Boolean ring.

Suppose you could find a ring $A$ of order $2^k$ with $n(A)$ ideals such that the number of prime factors (counted with multiplicity) of $n(A)$ is at least $k$. Suppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ has $2^{2l-k}p_1\dots p_l$ elements and ideals.

It seems quite hard to produce examples. I tried $A=\mathbb{F}_2\oplus V$, where $V$ is an $\mathbb{F}_2$-vector space and a square zero ideal in $A$, so the proper ideals of $A$ are the subspaces of $V$, but to the limit of my calculations $n(A)$ has far too few prime factors, and very rough heuristics suggest that this will continue.

The example I originally posted doesn't work.

Here are a couple of ideas that might lead to an example that's not a Boolean ring. I'll denote the number of ideals of a ring $A$ by $n(A)$.

(1) Suppose you could find a ring $A$ of order $2^k$ such that the number of prime factors (counted with multiplicity) of $n(A)$ is at least $k$. Suppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ has $2^{2l-k}p_1\dots p_l$ elements and ideals.

It seems quite hard to produce examples. I tried $A=\mathbb{F}_2\oplus V$, where $V$ is an $\mathbb{F}_2$-vector space and a square zero ideal in $A$, so the proper ideals of $A$ are the subspaces of $V$, but to the limit of my calculations $n(A)$ has far too few prime factors, and very rough heuristics suggest that this will continue.

(2) Here's a way of producing a ring $B$ where the number of prime factors of $n(B)$ is large compared to the number of prime factors of $|B|$.

Let $p$ be a prime such that $p+5=2^t$ is a power of $2$, and $t>4$. Then $B=\mathbb{F}_p[x,y]/(x^2,y^2)$ has $|B|=p^4$ and $n(B)=2^t$.

If you found a ring as in (1) but with $l$ slightly smaller than $k$, and where $p^4$ divides $n(A)$, then considering $A\times B$ and continuing as in (1) might work.

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Jeremy Rickard
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The example I originally posted doesn't work.

Here's an idea that might lead to an example that's not a boolean ring.

Let $$M=\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/2\mathbb{Z},$$ considered as a module for $\mathbb{Z}/16\mathbb{Z}$, and let $$A=\mathbb{Z}/16\mathbb{Z}\oplus M,$$ with the structure of aBoolean ring where $M$ is a square zero ideal. Then the proper ideals of $A$ are precisely the subgroups of $$2\mathbb{Z}/16\mathbb{Z}\oplus M\cong\mathbb{Z}/8\mathbb{Z}\times\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/2\mathbb{Z},$$ and according to Magma there are 1023 such subgroups (a minor miracle, as far as I can see!), so in total $A$ has 1024 ideals, and 512 elements.

So $$R=A\times\mathbb{F}_4$$ has 2048 ideals and 2048 elements.

You could make a similar example ifSuppose you could find anothera ring $A$ of order $2^k$ with $n(A)$ ideals such that the number of prime factors (counted with multiplicity) of the number of ideals of $A$$n(A)$ is at least $k$. You can then takeSuppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ has $2^{2l-k}p_1\dots p_l$ elements and ideals.

It seems quite hard to produce examples. I tried $A=\mathbb{F}_2\oplus V$, where $V$ is an $\mathbb{F}_2$-vector space and a square zero ideal in $A$, so the direct productproper ideals of $A$ with a collection of finite fields to makeare the numbersubspaces of elements equal$V$, but to the numberlimit of idealsmy calculations $n(A)$ has far too few prime factors, and very rough heuristics suggest that this will continue.

Here's an example that's not a boolean ring.

Let $$M=\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/2\mathbb{Z},$$ considered as a module for $\mathbb{Z}/16\mathbb{Z}$, and let $$A=\mathbb{Z}/16\mathbb{Z}\oplus M,$$ with the structure of a ring where $M$ is a square zero ideal. Then the proper ideals of $A$ are precisely the subgroups of $$2\mathbb{Z}/16\mathbb{Z}\oplus M\cong\mathbb{Z}/8\mathbb{Z}\times\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/4\mathbb{Z}\times\mathbb{Z}/2\mathbb{Z},$$ and according to Magma there are 1023 such subgroups (a minor miracle, as far as I can see!), so in total $A$ has 1024 ideals, and 512 elements.

So $$R=A\times\mathbb{F}_4$$ has 2048 ideals and 2048 elements.

You could make a similar example if you could find another ring $A$ of order $2^k$ such that the number of prime factors (counted with multiplicity) of the number of ideals of $A$ is at least $k$. You can then take the direct product of $A$ with a collection of finite fields to make the number of elements equal to the number of ideals.

The example I originally posted doesn't work.

Here's an idea that might lead to an example that's not a Boolean ring.

Suppose you could find a ring $A$ of order $2^k$ with $n(A)$ ideals such that the number of prime factors (counted with multiplicity) of $n(A)$ is at least $k$. Suppose $$n(A)=p_1\dots p_l,$$ with $l\geq k$, say.

Then $$A\times\mathbb{F}_{p_1}\times\dots\times\mathbb{F}_{p_l}\times\mathbb{F}_4^{l-k}$$ has $2^{2l-k}p_1\dots p_l$ elements and ideals.

It seems quite hard to produce examples. I tried $A=\mathbb{F}_2\oplus V$, where $V$ is an $\mathbb{F}_2$-vector space and a square zero ideal in $A$, so the proper ideals of $A$ are the subspaces of $V$, but to the limit of my calculations $n(A)$ has far too few prime factors, and very rough heuristics suggest that this will continue.

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Jeremy Rickard
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