In Chater 4 of Majid's A Quantum Groups Primer, he introduces an object he calls the ``automorphism quantum group'' M(A) associated to an algebra A. At the end of the chapter he makes a comment ``the role of such objects in number theory, however, is totally unexplored at the moment'', which was my queue to start searching for references. After I couldn't find any, I thought I'd take a stab at computing this object for some simple number-theory flavored objects. I haven't got anywhere yet, but let's take a look at what these objects are:
Let A be an algebra over a field k. We call B a comeasuring of A if it comes with an algebra map \beta: A \rightarrow A \otimes B, this is very similar to a coaction, except that we forget about a coalgebra structure. A morphism of comeasurings B_1 and B_2 is an algebra morphism \phi: B_1 \rightarrow B_2 such that \beta_2 = (I \otimes \phi) \circ \beta_1.
Hence we have a category where the objects are comeasurings of A and the arrows are comeasuring morphisms. When A is finite dimensional, we can prove that this category has an initial object, that is, an object M(A) such that for every other object B in the category, there exists one and only one unique morphism M(A) \rightarrow B.
The prove of the existence of M(A) for finite dimensional A is constructive, we won't go through the details here (see Majid's book), but we will state and use the result:
Let e_1,\ldots,e_n be a basis for A and let c_{ij}^k be the structure constants of A, id est, defined by
\displaystyle e_i e_j = \sum_{k=1}^n c_{ij}^k e_k
and define M'(A) as the free associative algebra k\langle t^1_1, \ldots t^1_n, \ldots t^n_1, \ldots t^n_n \rangle modulo the relations
\displaystyle \sum_{r=1}^n c_{ij}^r t^k_r = \sum_{r,s=1}^n c_{rs}^k t^r_i t^s_j
for i,j,k=1 to n The initial object M(A) is M'(A) modulo the additional relations t^1_1 =1, t^i_1 =0 for 1 < i \leq n. The initial map \beta_{M(A)}: A \rightarrow A \otimes M(A) is:
\displaystyle \beta_{M(A)}(e_i) = \sum_{r=1}^n e_r \otimes t^r_i
The reader can verify that this \beta_{M(A)} is an algebra map and thus verify that this is indeed a comeasuring. To show that it's initial, we need to show that for any other comeasuring B with map \beta: A \rightarrow A \otimes B, there exists a unique algebra morphism \pi: M(A) \rightarrow B. Let b^i_j = \sum_r e_r \otimes t^r_i and observe that \sum_r c_{ij}^a b^k_r = \sum_{r,s} c_{rs}^k b^r_i b^s_j, so we can let \pi(t^i_j) = b^i_j and extend this as multiplicative map. Uniqueness follows from having finite dimension and the by noting that another morphism \pi' must obey the same relations above on the same image of \beta.
The algebra M(A) is what Majid calls the automorphism quantum group. It is a bialgebra. I won't prove it, but you can see that it has a coproduct, et cetera by noting that M(A) \otimes M(A) is a comeasuring with the map (\beta_{M(A)} \otimes I) \circ \beta_{M(A)}, hence we have a unique morphism \Delta: M(A) \rightarrow M(A) \otimes M(A) by the initial object condition. The counit pops out in a similar way. The coproduct on the generators is
\displaystyle \Delta t^i_j = \sum_{r=1}^n t^i_r \otimes t^r_j
As an example of this nonsense, let m(x) \in \mathbb{F}_p[x] be an irreducible polynomial of degree n over the field with p elements, and let F_m by the corresponding field extension (which is an algebra over \mathbb{F}_p). For each other irreducible polynomial g of degree n, we have an isomorphic field F_g. Each F_g is also an object in the category of comeasurings of F_m, as the isomorphism F_m \rightarrow F_g will give rise to an algebra map F_m \rightarrow F_m \otimes F_g. M(F_m) is a n^2 -n dimensional algebra in this category that has a unique isomorphism to each F_g, hence M(F_m) and its maps to the F_g's would capture the arithmetic of each other field extension of the same degree. I hope that I can use this object in my program for the quantum de rham cohomology over finite fields. One obvious task to describe how the various M(F_g) relate? The relations will be governed by a rearrangement of the structure constants. Of course they should all be isomorphic, and each M(F_g) exists in the other's M(F_m) category of comeasurings, too...(I suspect my ideas are a bit vacuous.)
In the case of \mathbb{F}_2, consider the extension \mathbb{F}_4 \cong \mathbb{F}_2[x]/(x^2 +x + 1) as a finite dimensional algebra. The structure constants are easy to compute, and we can easily see that the dimension of M(\mathbb{F}_4) is 2. Let's call its generators \alpha and \beta, then using the above formulas we have that \beta_{M(\mathbb{F}_4)}(x) = x \otimes \alpha + x \otimes \beta and the relations are 1 + \alpha = \alpha^2 + \beta^2 and [\alpha, \beta] = \beta^2 + \beta. This algebra is clearly a lot more complicated than the simple field extension. One should note that there are no other field extensions of degree 2 of \mathbb{F}_2.
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