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Given some algebra $\mathcal{A}$ and $q \in \mathbb{C}$, we say that a matrix $M \in \mathrm{Mat}(n \times n ; \mathcal{A})$ is a quantum matrix in $\mathrm{Mat}_q(n \times n)$ iff the following conditions hold for any submatrix $\begin{pmatrix}a&b\\c&d\end{pmatrix}$ of $M$:

\begin{equation} \label{eq:commrels} \begin{split} ca = q \cdot ac \,\,\,,\,\,\, db = q \cdot bd \,\,\,,\,\,\, ad - da = q^{-1} \cdot cb - q \cdot bc \\ ba = q \cdot ab \,\,\,,\,\,\, dc = q \cdot cd \,\,\,,\,\,\, ad - da = q^{-1} \cdot bc - q \cdot cb \\ bc = cb \,\,\,,\,\,\, ad - da = (q^{-1}-q) bc. \end{split} \end{equation}

Note that the third line of equalities follows from the first two (if $q \neq \pm 1$). The first line can be derived by considering a 'quantum vector space' spanned by 'vectors' $\begin{pmatrix} x_1 \\ \vdots \\ x_n \end{pmatrix}$ and its 'dual' spanned by 'covectors' $( x'_1 \cdots x'_n )$, subject to commutation relations $x_j x_i = q x_i x_j$ and $x'_j x'_i = q x'_i x'_j$ for $i<j$ and for which all entries commute with the entries of $M$. Considering the transformed vectors \begin{equation} \begin{pmatrix} \tilde{x}_1 \\ \vdots \\ \tilde{x}_n \end{pmatrix} = M \begin{pmatrix} x_1 \\ \vdots \\ x_n \end{pmatrix} \,\,\, \text{i.e.} \,\,\, \tilde{x}_i = \sum_{j} M_{i,j} x_i \quad,\quad ( \tilde{x}'_1 \cdots \tilde{x}'_n ) = ( x'_1 \cdots x'_n ) M \,\,\, \text{i.e.} \,\,\, \tilde{x}'_j = \sum_{j} x'_i M_{i,j} \end{equation} Then the first line of the commutation relations can be derived from imposing that $\tilde{x}_j \tilde{x}_i = q \tilde{x}_i \tilde{x}_j$ for $i<j$ and the second line can be derived from $\tilde{x}'_j \tilde{x}'_i = q \tilde{x}'_i \tilde{x}'_j$ for $i < j$.

Standard treatments of 'quantum linear algebra' (see e.g. Chapter 4.5 in Shahn Majid's "Foundations of Quantum Group Theory") usually work with the q-commuting 'coordinate rings' $\mathbb{C}_q[x_1,\cdots,x_n] = \mathbb{C} \langle x_1,\cdots,x_n \rangle/\{x_j x_i = q x_i x_j, i < j\}$ rather than a vector space itself. And comparisons to regular linear algebra are made for commuting coordinate rings $\mathbb{C}[x_1,\cdots,x_n]=\mathbb{C} \langle x_1,\cdots,x_n \rangle /\{x_j x_i = x_i x_j, i < j\}$.

A rough version of my question is: "Are there more invariant definitions of quantum vector spaces, quantum linear maps, quantum eigenvectors, etc that don't rely on the coordinate ring itself?".

There are some more related (possibly more pointed) questions below. Any answers or intuitions for any of these questions would be appreciated.

I was told that the standard way to relate the classical coordinate ring $\mathbb{C}[x_1,\cdots,x_n]$ to a vector space is to consider the 'prime spectrum' $\mathrm{Spec}(\mathbb{C}[x_1,\cdots,x_n])$ of $\mathbb{C}[x_1,\cdots,x_n]$, which is the set of prime ideals of the ring. In this case, the prime ideals are of the form $\langle (x_1-a_1) \cdots (x_n-a_n) \rangle$ for $a_i \in \mathbb{C}$ which are in bijection with $\mathbb{C}^n$. I heard that $\mathrm{Spec}(\mathbb{C}_q[x_1,\cdots,x_n])$ should be the 'quantum vector space' we're looking for.

This leads to the question: "Is there a direct, intrinsic vector space structure on $\mathrm{Spec}(\mathbb{C}[x_1,\cdots,x_n])$ that generalizes for $q \neq 1?$".

A (maybe vague) reverse question would be: "How much linear algebra can be recovered by just considering the coordinate ring $\mathbb{C}[x_1,\cdots,x_n]$?".

The reason I'm wondering this is motivated by a hypothetical notion: "Is there a notion of a 'quantum vector bundle' and a 'quantum flat connection' on surfaces?" (an idea that Witten mused should be related to quantum groups in his famous "Quantum Field Theory and the Jones Polynomial" paper).

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  • $\begingroup$ You already mentioned Majid's FQGT book, but you might also like the more recent Beggs-Majid, "Quantum Riemannian Geometry" (link.springer.com/book/10.1007/978-3-030-30294-8). $\endgroup$ Commented Mar 3, 2023 at 7:22
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    $\begingroup$ Taking Spec of the coordinate ring puts you in the world of affine varieties: it would be more common to call Spec of the polynomial ring "affine space" and correspondingly Spec of the quantum one "quantum affine space". This might help with searching in and/or reading the literature. $\endgroup$ Commented Mar 3, 2023 at 7:25
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    $\begingroup$ For your first question, I think it's fair to say that the conventional take is "no" - in noncommutative geometry, there is no actual space, just the algebra. So one dualises notions from the geometry side over to the algebra (i.e. coordinate ring) and makes the corresponding construction there. $\endgroup$ Commented Mar 3, 2023 at 7:28
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    $\begingroup$ For your second, either you've answered your own question in the previous paragraph or you have in mind a different meaning of "generalises". Away from q=1 (and especially for roots of unity) quantum affine space is quite different from the commutative one, and I don't know a way around that. (And it's a matter of taste whether you think this is a feature or a bug.) $\endgroup$ Commented Mar 3, 2023 at 7:30
  • $\begingroup$ Thanks these are very helpful, I'm looking into these resources now. My question was more along the lines of "Is there an intrinsic vector space or affine structure on $\mathbb{C}[x_1,\cdots,x_n]$ that doesn't rely on passing to $\mathbb{C}^n$?". $\endgroup$
    – Joe
    Commented Mar 3, 2023 at 21:09

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