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Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

2 votes
1 answer
223 views

Are non-maximal orders in number fields Grothendieck rings?

Recall that a ring homomorphism A->B is geometrically regular if for all primes p of A, the fiber of B over p is geometrically regular over k(p). A Grothendieck ring (or, G-ring) is one for which A_p- …
Benjamin Antieau's user avatar
5 votes

On a morphism from the Brauer group to the Picard group

Here is another way to see that the given map should be zero. It corresponds to a map of sheaves of grouplike $\mathbb{E}_\infty$-spaces $K(\mathbb{G}_m,2)\rightarrow K(\mathbb{G}_m,1)$. All such maps …
Benjamin Antieau's user avatar
-1 votes

Can a quotient ring R/J ever be flat over R?

I believe that one can use an Eilenberg swindle-type argument. Let N denote the positive integers. Let A=Z[N] denote the Z-polynomial ring on N. Create a surjective morphism A->A by sending x_1 to zer …
Benjamin Antieau's user avatar
-1 votes

Can a quotient ring R/J ever be flat over R?

Here is a better (read: correct) answer than what I tried above. There is a ring we all known and love: the ring of dual numbers, k[e]/(e^2). I claim that k=(k[e]/(e^2))/(e) is a projective k[e]/(e^2 …
Benjamin Antieau's user avatar