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We prove that F-injectivity localizes, descends under faithfully flat homomorphisms, and ascends under flat homomorphisms with Cohen–Macaulay and geometrically F-injective fibers, all for arbitrary Noetherian rings of prime characteristic. As a consequence, we show that the F-injective locus is open on most rings arising in arithmetic and geometry. As a geometric application, we prove that over an algebraically closed field of characteristic p > 3, generic projection hypersurfaces associated to suitably embedded smooth projective varieties of dimension ≤5 are F-pure, and hence F-injective. This geometric result is the positive characteristic analogue of a theorem of Doherty. 

An excellent ring of prime characteristic for which the Frobenius map is pure is also Frobenius split in many commonly occurring situations in positive characteristic commutative algebra and algebraic geometry. However, using a fundamental construction from rigid geometry, we show that excellent $$F$$-pure rings of prime characteristic are not Frobenius split in general, even for Euclidean domains. Our construction uses the existence of a complete non-Archimedean field $$k$$ of characteristic $$p$$ with no nonzero continuous $$k$$-linear maps $$k^{1/p} \to k$$. An explicit example of such a field is given based on ideas of Gabber, and may be of independent interest. Our examples settle a long-standing open question in the theory of $$F$$-singularities whose origin can be traced back to when Hochster and Roberts introduced the notion of $$F$$-purity. The excellent Euclidean domains we construct also admit no nonzero $$R$$-linear maps $$R^{1/p} \rightarrow R$$. These are the first examples that illustrate that $$F$$-purity and Frobenius splitting define different classes of singularities for excellent domains, and are also the first examples of excellent domains with no nonzero $$p^{-1}$$-linear maps. The latter is particularly interesting from the perspective of the theory of test ideals.