REVIEW 2 major objections 5 minor 1 cited by
Sharp Bounds for Totally Invariant Cycles of Projective Varieties
T0 review · 2 major / 5 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read For any self-map of projective n-space of degree q>1, the number of totally-invariant codimension-r cycles is at most the binomial coefficient n+1 choose r, and this bound is sharp.
desk verdict Resolves Favre's question with a sharp P^2 bound and proves a clean binomial bound in all codimensions for P^n; the main proof is solid, with one load-bearing unproved characteristic-free lemma to fix. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The paper's working object is the normalized trace of a finite locally free endomorphism. When a reduced subscheme Z satisfies (h^{-1}(Z))_red = Z, Lemma 2.1 splits the inclusion I_Z -> h_*I_Z by d^{-1}Tr_h, making H^i(X, I_Z ⊗ A) a direct summand of H^i(X, I_Z ⊗ h^*A). Combined with Lemma 2.2 (iterates of an int-amplified map make h^*L ⊗ L^{-1} ample) and uniform Fujita vanishing, this yields the vanishing H^i(X, I_Z ⊗ L^t) = 0 for all i,t>0 (Proposition 2.3). Successive very ample hyperplane sections then express the degree of Z as the length of a zero-dimensional scheme, bounded by the Hilbert function of a complete intersection, which in P^n evaluates to the binomial coefficient.
What would settle it
An endomorphism of P^n of degree q>1 invertible in k with total degree of its totally-invariant codimension-r cycles exceeding binom(n+1,r), or a smooth projective X with an int-amplified endomorphism of degree invertible in k where the Hilbert-function inequality in Theorem 1.1 fails, would refute the central claim. A simpler place to look: verify Lemma 2.2 in characteristic p>0 with p ∤ deg(f); if the ampleness of (f^s)^*L ⊗ L^{-1} fails for some large s, Proposition 2.3 collapses.
Extended reading notes
Core claim
The central claim is Theorem 1.3: for an endomorphism f of P^n with f^*O(1) = O(q), q>1 invertible in the field, the set of totally-periodic prime cycles of codimension r is finite and the sum of their degrees is at most binom(n+1,r), so there are at most binom(n+1,r) of them. This bound is sharp, realized by the coordinate power map, whose coordinate linear subspaces are the extremal cycles. The same mechanism yields a general bound on any smooth projective variety X: for an ample line bundle L, the total L-degree of codimension-r totally-periodic cycles is controlled by a finite difference of the Hilbert function of L, given in Theorem 1.1. The proof uses a normalized trace to split the id
Load-bearing premise
The proof assumes that the cone-theoretic eigenvalue criterion for int-amplified endomorphisms, which the cited reference states in characteristic zero, remains valid over any algebraically closed field; the paper asserts this without proof.
Editorial extensions
If this is right
- Favre's question for P^2 is answered: |T_∞(f)| ≤ 3 for every degree-q>1 endomorphism of the projective plane, with equality for the coordinate power map.
- In every codimension 1 ≤ r ≤ n of P^n, the coordinate power map attains the bound, so the binomial constants cannot be improved; any endomorphism achieving the full binomial polynomial must, after an iterate, be projectively conjugate to that map.
- For completely ramified points, when q ≥ n+1, the paper obtains the sharp bound n+1 and projective linear independence, improving the previous plane-curve bound of nine to three for cubic and higher degree maps of P^2.
- The general Hilbert-function bound applies to all smooth projective varieties admitting int-amplified endomorphisms of degree invertible in k, giving degree and cardinality bounds that are uniform over all such endomorphisms once L and a are fixed.
- Applications include: blow-ups of P^n along smooth codimension-two totally-invariant cycles are Fano; log canonical centers of invariant boundaries satisfy the same binomial bound; and the number of contractible extremal rays is linearly controlled.
Reading between the lines
- If the paper's one-sentence characteristic-free assertion of the cone-theoretic criterion is correct, Theorem 1.1's bounds hold verbatim over every algebraically closed field of characteristic not dividing deg(f); checking a single int-amplified map over F_p whose degree is coprime to p would be a cheap test.
- Remark 4.2 shows that the polynomial identity Φ = (1+t)^{n+1} does not characterize P^n as an underlying variety; a natural next step would be to classify all smooth projective X admitting an int-amplified f with maximal totally-periodic polynomial.
- Because the proof of Proposition 2.3 only needs iterates to amplify L, the same Hilbert-function bound should apply to any self-map satisfying that amplification condition, not only to int-amplified endomorphisms.
- The sharp bound on completely ramified points when q ≥ n+1 suggests that for low-degree maps (q < n+1) the linear-independence statement may fail; computing examples in degree 2 on P^2 would map the boundary of the sharp result.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves explicit upper bounds on the total degree and number of totally-periodic prime cycles of codimension r for int-amplified endomorphisms of smooth projective varieties, under the assumption that deg(f) is invertible in the ground field. The main mechanism is a normalized trace splitting for ideal sheaves of totally periodic subschemes (Lemma 2.1), followed by uniform Fujita-type vanishing (Proposition 2.3) and a finite-difference/Hilbert-function computation. In projective space the bound becomes binom(n+1,r), with sharpness attained by the coordinate power map; this answers Favre's question for P^2 by giving |T_∞(f)|≤3. The paper also proves toric analogues, studies the totally-periodic polynomial, and derives applications to completely ramified points, log canonical centers, and contractible extremal rays.
Significance. If the proof is completed at the points noted below, the paper is a significant contribution. Theorem 1.1 gives a closed-form, parameter-free bound that is uniform over all int-amplified endomorphisms once L and a are fixed, and the projective-space specialization is sharp with an explicit extremal example. The method is transparent, uses standard intersection-theoretic and vanishing-theoretic tools, and reproduces known codimension-one results as special cases. The paper also gives falsifiable numerical predictions and explicit constructions, including the coordinate power map and the F_1 example in Remark 4.2. The positive-characteristic scope is an intended feature, so the missing justification for the cone-theoretic criterion is a genuine gap rather than a stylistic issue.
major comments (2)
- [Section 2, Lemma 2.2] This lemma is the only bridge between the int-amplified hypothesis and the numerical eigenvalue statement used in Proposition 2.3. The cited criterion [26, Theorem 1.1 and Remark 1.2] is stated in characteristic zero, and the manuscript's one-sentence assertion that the proof is 'purely cone-theoretic' and works over any algebraically closed field is not a proof. Since Theorem 1.1 is stated over an arbitrary algebraically closed field and only deg(f) invertible is imposed, positive characteristic is explicitly in scope. If this characteristic-free extension cannot be justified, Proposition 2.3, and hence Theorem 1.1 and the P^n bounds, do not follow in positive characteristic. Please provide a self-contained proof or a citation to a characteristic-free statement.
- [Section 4, proof of Proposition 1.5(4)] As printed, the proof uses the equality (f^a)_*V = q^{ar}V and defines the Chow morphism τ_{r,e,a} with target Chow_{n-r,q^{ar}e}. For an integral V of codimension r (dimension m=n-r) that is totally invariant, flat pushforward satisfies (f^a)_*[V] = q^{a m}[V] = q^{a(n-r)}[V], not q^{ar}[V]; for points (r=n) one gets 1·[V], not q^{an}[V]. The factor q^{ar} is the correct factor for flat pullback (f^a)^*[V]. Thus the displayed equality and the subsequent properness argument for C_{r,e,a} are invalid as written. If the intended operation is pullback, replace the pushforward stars with pullback stars throughout and check the parametrized family; if pushforward is intended, the degree factor and the target Chow degree must be changed. This does not affect Theorems 1.1–1.4, but it is load-bearing for the generic left-equality statement in Proposition 1.5(4).
minor comments (5)
- [Section 4, Proposition 1.5(3)] The expression Φ_{X,f}(t)-t^{n+1} includes the constant term 1. This is correct for the product inequality in (3), but it is easy to misread; a parenthetical reminder that the constant term is retained would help.
- [Section 3, Proposition 2.3 proof] The sentence 'The divisor h*H−H is a sum of pullbacks of the ample divisor f*H−H' is terse. Since h=f^b, the telescoping sum is immediate, but spelling it out would improve readability.
- [Remark 3.2] For finite fields, 'q is invertible in k' means q is not divisible by p. Stating this explicitly would remove ambiguity in the example k=F_p.
- [Section 5.3, Corollary 5.6] The identity 'projection formula gives c deg(f|_Z)=deg(f)' is stated without derivation. A one-line computation of the cycle classes involved would help the reader verify the contradiction.
- [General] There are minor typographical issues, including inconsistent spacing in formulas such as T^r_∞(X,f) and Chow_{n-r,e}(P^n). These should be cleaned up during revision.
Circularity Check
No circularity: the derivation is self-contained; the only flagged gap is an external-support issue in Lemma 2.2, not a circular reduction.
full rationale
I walked the proof chain: Theorem 1.3 uses Theorem 1.1; Theorem 1.1 is proved from Proposition 2.3; Proposition 2.3 is proved from Lemma 2.1 (trace splitting, proved locally) and Lemma 2.2. Lemma 2.2 invokes Meng's cone-theoretic eigenvalue criterion [26, Theorem 1.1 and Remark 1.2] and explicitly asserts, without proof, that the criterion extends from characteristic zero to any algebraically closed field. This is a missing verification / potential correctness gap, but it is not circularity: the cited criterion is an external theorem, not an input equivalent to the paper's conclusion, and the paper's derivation from it is explicit. The bound B_{X,L,a,r} is a computed alternating sum of Hilbert functions, not a fitted parameter, and no quantity from T^r_∞(f) is inserted into the bound. Sharpness is demonstrated by the coordinate power map, whose totally invariant linear subspaces are counted directly. I found no self-definitional step, no fitted-input-called-prediction, no load-bearing self-citation, and no renaming of a known result as a new derivation. The unsupported characteristic-free extension in Lemma 2.2 should be verified, but it does not make the argument circular.
Assumptions & free parameters
assumptions (7)
- domain assumption Meng's cone-theoretic eigenvalue criterion for int-amplified endomorphisms holds over any algebraically closed field.
- domain assumption Fujita's uniform vanishing theorem (Keeler's form) gives H^i(X,F⊗L^m⊗N)=0 for m>>0 and N nef.
- domain assumption Fakhruddin's theorem [12, Theorem 1.2] that a generic degree-q endomorphism of P^n has no positive-dimensional totally invariant subvarieties.
- domain assumption Broustet-Höring [3, Theorem 1.4 and Lemma 2.10] on non-log-canonical loci and log canonical centers of invariant boundaries.
- domain assumption Meng-Zhang [27, Lemma 2.6] states that for an int-amplified endomorphism, a proper totally-invariant subvariety has pushforward c[Z] with c>1.
- standard math Chow scheme representability and flat pullback properties (Kollár, Stacks Project).
- standard math Miracle flatness: a finite morphism from a smooth variety is locally free.
Cite this review
Pith. "Pith review of Sharp Bounds for Totally Invariant Cycles of Projective Varieties." pith.science (2026). https://pith.science/paper/TUAXY6LW
@misc{pith2026260727738,
author = {Pith},
title = {Pith review of: Sharp Bounds for Totally Invariant Cycles of Projective Varieties},
year = {2026},
howpublished = {\url{https://pith.science/paper/TUAXY6LW}},
note = {Machine review of arXiv:2607.27738}
}
abstract
Let $X$ be a smooth projective variety, $f:X\to X$ an int-amplified endomorphism, and $L$ any ample line bundle on $X$. We prove that, in every codimension, the total degree of prime cycles that become totally invariant under an iterate of $f$ satisfies an explicit Hilbert-function bound on their total $L$-degree. In particular, when $X=\mathbf{P}^n$, the number of totally invariant prime $(n-r)$-cycle is bounded by $\binom{n+1}{r}$, and this bound is optimal.
Forward citations
Cited by 1 Pith paper
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Log Calabi--Yau structure for endomorphisms on $\mathbf{P}^n$
For every q-polarized endomorphism of P^2, some iterate (P^2, R_{f^s}/(q^s-1)) is log canonical, settling Gongyo's conjecture for smooth projective surfaces.
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