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In the fields of representation theory and quantum algebra, we often start with some $\mathbf{C}$-algebra and study it's quantization as an algebra over $\mathbf{C}(q)$, using the algebra structure to twist the multiplication. But what is $q$? Taking the calculus-flavored approach like in Quantum Calculus by Kac and Cheung, $q$ is just some indeterminate complex number: we specialize $q \to 1$ to get our original un-quantized object back, or we need $|q|<1$ for certain series to converge, or we specialize $q$ to some root of unity to make $q$ have finite order.

But in other applications, like counting subspaces of a vector space over $F_q$, or defining the structure constants of the Hall algebra of a $F_q$-linear category, we use the same $q$-calculus framework, but we are now thinking of $q$ as a power of a prime.

How do we consolidate the calculus interpretation of $q$ as an element of our base field with the combinatorial interpretation of $q$ as a prime power?

Mike Pierce
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  • We consolidate it by making $q$ an indeterminate, which is what they've already done. – Matt Samuel Oct 21 '19 at 16:52
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    @MattSamuel But, like, there's got to be some meaningful connection between the two interpretations of $q$, right? It must be more than a happy accident that we get neat combinatorial facts from considering $q$ as a prime power. – Mike Pierce Oct 21 '19 at 16:53
  • This is an excellent question. Also look up the so-called "cyclic-sieving phenomena," whose explanation at least in some cases is isotypical decompositions (a la representation theory), IIRC. – anon Mar 09 '20 at 00:03

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