1. $\mathrm{FI}$-modules and tca’s
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Nate Harman’s question on lifting $\mathrm{FI}$-modules from $\mathbb{C}$ to $\mathbb{Z}$
Problem 1.1.
[Nate Harman] Given a map $\phi_{\mathbb{Q}} \colon V_{\mathbb{Q}} \to W_{\mathbb{Q}}$ of finitely generated $\mathrm{FI}$-modules over $\mathbb{Q}$ (or may be over an algebraically closed field), and choices $V, W$ of lattices in $V_{\mathbb{Q}}$ and $W_{\mathbb{Q}}$. If $p$ is sufficiently large, is $ V \otimes_{\mathbb{Z}} \mathbb{F}_p \to W \otimes_{\mathbb{Z}} \mathbb{F}_p$ independent of the choice of $V$ and $W$ up to isomorphism? -
Noetherianity of $\mathrm{FI}$-algebras
Problem 1.2.
Let $V_n = \mathbf{k}[x_1 , x_2, \ldots x_n]$. Find a concrete example of a sub-$\mathrm{FI}$-algebra which is not noetherian. Is there a version for finitely presented modules? Does it depend on characteristic? -
Classify torsion free injectives
Problem 1.3.
[John Wiltshire-Gordon] What’s the injective hull of the trivial $\mathrm{FI}$-modules ($V_n = \mathrm{triv}_n$ for each $n$)? In general, classify all indecomposable injectives. Is it true that these indecomposable injectives are degree-wise finitely generated (same question over a more general combinatorial category). -
Highest weight category
Problem 1.4.
[R. Nagpal] Working over a field. Is the Serre quotient $\mathrm{Mod}_{\mathrm{FI}}/\mathrm{Mod}_{\mathrm{FI}}^{\mathrm{tors}}$ a highest weight category? -
Noetherianity of degree two tca’s
Problem 1.5.
- {T. Church} Is $\mathrm{Sym}(\mathrm{Sym}^2)$ noetherian over $\mathbb{Z}$ $($or over a general noetherian ring$)$?
- {J. Wilson} Same question for $\bigwedge(\mathrm{Sym}^2)$.
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Bounds on syzygies of modules over degree two tca’s
There are no linear bounds, that is, regularity is infinite. As an example, take $A$ modulo a determinantal ideal (see [section 6.3, W]). The calculations have been done in characteristic zero only though.Problem 1.6.
[J. Miller] Let $A = \mathrm{Sym}(\mathrm{Sym}^2)$ or $\bigwedge(\mathrm{Sym}^2)$, and let $M$ be an $A$-module generated in degree $d$ and related in degree $r$. Are there any good bounds on the syzygies? In other words, bound generators of the terms of the free resolutions. Can start with characteristic $0$.
[W] Jerzy Weyman, Cohomology of Vector Bundles and Syzygies, Cambridge University Press, Cambridge, 2003. -
Hilbert series of modules over degree two tca’s
Concretely, let \[ \mathrm{H}_m(t) = \sum_{|\lambda| = n} \dim(M_{2\lambda}) t^{2n}. \] Is $\mathrm{H}_m(t)$ algebraic?Problem 1.7.
[A. Snowden] What can we say about Hilbert series of modules over $A = \mathrm{Sym}(\mathrm{Sym}^2)$. More precisely, take $M$ to be $A$ modulo a determinantal ideal. Is it true that its $($non-exponential$)$ Hilbert series is algebraic? Assume characteristic $0$.
Note that $\mathrm{H}_m(t)$ is the ordinary Hilbert series of $A$ modulo the $m$th determinantal ideal. If $m = \infty$, then $\mathrm{H}_m(t)$ is not algebraic. $H_2(t)$ is algebraic and is closely related to Catalan numbers. -
$q$-tca’s
Berenstein–Zwicknagl define quantum analog of $\mathrm{Sym}(\mathbf{S}_{\lambda})$ in [BZ]. For small $\lambda$, this behaves like the classical case, but for large $\lambda$ (and $q$ generic) its isotypic decomposition has smaller multiplicities than the classical case.$\mathrm{Sym}(\mathbf{S}_1 \oplus \mathbf{S}_1)$ already smaller than classical case, question is interesting here. (Already known for $\mathrm{Sym}(\mathbf{S}_1)$.)Problem 1.8.
[D. Barter] Are modules over this algebra $($as $q$-tca’s$)$ equivalent to modules in the classical case $($as tca’s$)$?
[BZ] Arkady Berenstein, Sebastian Zwicknagl, Braided symmetric and exterior algebras, Trans. Amer. Math. Soc. 360 (2008), no. 7, 3429–3472
Cite this as: AimPL: Representation stability, available at http://aimpl.org/repnstability.