The uniform Galois-group conjecture for reductions of D-finite series

Let f(x)Q[[x]]f(x)\in\mathbb{Q}[[x]] be a DD-finite series and let nn be the minimal order of a differential operator that annihilates f(x)f(x). Let KK be a number field and let (G1,,Gt)(G_1,\ldots,G_t) be a family of subgroups of GLn(K)\operatorname{GL}_n(K). For a prime p\mathfrak p of KK, write O(p)\mathcal{O}_{(\mathfrak p)} for the localization of the ring of integers of KK at p\mathfrak p and kpk_\mathfrak p for its residue field. For an algebraic reduction f(x)modpf(x)\bmod p, its residual splitting field is the intersection of its splitting field in Fp(x)sep\mathbb{F}_p(x)^{\mathrm{sep}} with Fp\overline{\mathbb{F}}_p. Uniform Galois-group conjecture. There exist a number field KK and a family (G1,,Gt)(G_1,\ldots,G_t) of subgroups of GLn(K)\operatorname{GL}_n(K) such that, for almost all rational primes pp for which f(x)modpf(x)\bmod p is well defined and algebraic over Fp(x)\mathbb{F}_p(x), and for any prime p\mathfrak p of KK above pp, there exists i{1,,t}i\in\{1,\ldots,t\} such that the residual splitting field of f(x)modpf(x)\bmod p is contained in kpk_\mathfrak p and

Gal(f(x)kp(x))image(GiGLn(O(p))GLn(kp)).\operatorname{Gal}\bigl(f(x)\mid k_\mathfrak p(x)\bigr)\simeq \operatorname{image}\bigl(G_i\cap\operatorname{GL}_n(\mathcal{O}_{(\mathfrak p)})\to\operatorname{GL}_n(k_\mathfrak p)\bigr).

The conjecture predicts that only finitely many linear groups govern the Galois groups of algebraic reductions, after allowing a fixed number field and residue-field reduction. The paper presents examples supporting this uniformity, but the general assertion remains open.

Sources & referencesView supporting material

Primary source

Xavier Caruso, Florian Fürnsinn and Daniel Vargas-Montoya, “Galois groups of reductions modulo p of D-finite series”, arXiv:2504.09429 (2025).

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