REVIEW 5 major objections 4 minor 30 references
Automorphisms of very general blow up
T0 review · 5 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read For every projective variety of dimension at least two that is not a rational surface, blowing up sufficiently many very general points leaves no nontrivial automorphism.
desk verdict New and likely correct generalization of the P^2/P^3 blow-up rigidity result, with a clean fixed-point criterion; two fixable but load-bearing indexing and justification gaps in the ruled-surface case. 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 load-bearing tool is Theorem 3.1, a rigidity lemma: for a normal projective variety $X$ and a proper closed subset $Y$, once $r>\dim\operatorname{Aut}^0(X)+1$ (or $r>2\dim\operatorname{Aut}^0(X)+1$ in dimension $1$), no nontrivial automorphism can send a very general $r$-tuple of points into itself together with $Y$. Here 'very general' means outside a countable union of proper subvarieties of $X^r$. For ruled surfaces, the central mechanism is the elementary transformation $\operatorname{Elm}(X,p)$, which replaces a minimal ruled surface $\mathbb{P}(E)$ by $\mathbb{P}(F)$, where $F$ is the kernel of a surjection to skyscraper sheaves; Proposition 4.4 then shows that any involution over the base curve must preserve one of the blown-up fibres. Lemma 4.2 supplies the needed dimension bound for the family of endomorphisms $\eta$ satisfying $\eta^2 = s\cdot\mathrm{id}$.
What would settle it
Take a concrete non-rational ruled surface such as $X=\mathbb{P}^1\times E$ with $E$ an elliptic curve, choose $r$ successively larger, and compute $\operatorname{Aut}(\operatorname{Bl}_{p_1,\dots,p_r}X)$ for a very general $r$-tuple; the paper's proof predicts the group is trivial for all sufficiently large $r$, so any non-identity automorphism found would contradict Theorem A.
Extended reading notes
Core claim
The central claim is Theorem A: if $X$ is a projective variety of dimension at least $2$ and $X$ is not a rational surface, then for all sufficiently large $r$ and very general points $p_1,\dots,p_r\in X$, the blow-up of $X$ at these points has no nontrivial automorphism. The proof is divided into three regimes: dimension at least three, non-uniruled surfaces, and uniruled non-rational surfaces. In each regime, an automorphism of the blow-up is forced to descend to an automorphism of a simpler model, and a dimension-counting lemma shows that such a symmetry cannot preserve a very general $r$-tuple once $r$ is large enough.
Load-bearing premise
For surfaces, the proof assumes that every symmetry of the blown-up surface can be pushed down to a symmetry of a simplified version of the surface (or of its base curve), rather than only to a more general birational map; if that descent fails, the argument breaks.
Editorial extensions
If this is right
- Every projective variety of dimension at least three becomes asymmetric after blowing up sufficiently many very general points, because the rational-surface exception is automatic only in dimension two.
- For any non-rational projective surface, the blow-up at sufficiently many very general points has trivial automorphism group, so its automorphism group scheme is a single point.
- The application gives a smooth projective surface with trivial automorphism group that still admits infinitely many elliptic bundle structures distinct modulo automorphisms, showing that non-minimal surfaces can carry infinitely many such structures.
- The remarks show that the 'very general' hypothesis is essential: for abelian varieties, suitably chosen general points can be fixed by a nontrivial automorphism, so the blow-up can retain symmetry.
Reading between the lines
- Beyond the paper, the explicit bound $r_0=\dim\operatorname{Aut}^0(X)+1$ from Theorem 3.1 suggests that for a fixed variety one could in principle compute the smallest number of points guaranteeing asymmetry, a quantitative question the paper does not pursue.
- Beyond the paper, the elementary-transformation machinery may extend from ruled surfaces to higher-rank projective bundles, where the same dimension-counting strategy could kill automorphisms over the base curve or base variety.
- Beyond the paper, closing the rational-surface gap would make the statement uniform across all projective surfaces; the known $\mathbb{P}^2$ case indicates the missing ingredient is a descent argument for automorphisms landing on $\mathbb{P}^2$ or on Hirzebruch surfaces.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves Theorem A: if X is a projective variety of dimension at least 2 and X is not a rational surface, then for all sufficiently large r, the blow-up of X at r very general points has trivial automorphism group. The proof combines a dimension-counting statement for automorphisms preserving a finite set up to a fixed closed subset (Theorem 3.1), a lemma on (-1)-hypersurfaces, and a reduction of the non-rational ruled-surface case to elementary transformations of ruled surfaces. An application constructs a smooth projective surface with trivial automorphism group and infinitely many elliptic bundle structures.
Significance. Theorem A is a natural extension of known results for P^2 and P^3, and the elementary-transformation analysis in Section 4 together with the automorphism-group dimension count in Theorem 3.1 are substantive and potentially reusable tools. The application in Section 6 is concise and interesting. If the gaps identified below are repaired, the paper is a solid contribution to the study of generic triviality of automorphism groups under birational transformations.
major comments (5)
- [Section 2, Lemma 2.2] The proof of Lemma 2.2 contains a false assertion: 'As E_2 is isomorphic to P^{n-1} and f|_{E_2} is nonconstant, f|_{E_2} must be finite.' A nonconstant morphism from P^{n-1} need not have finite fibres; for example, if E_1 and E_2 meet, then f|_{E_2} contracts the positive-dimensional divisor E_1 intersect E_2 to the blown-up point. A correct proof should use that f|_{E_2} is the blow-up of the image divisor at the blown-up point and that E_2 is isomorphic to P^{n-1}; this forces the image to be a P^{n-1} not containing the point, so the two (-1)-hypersurfaces are disjoint. Since Lemma 2.2 underpins the descent argument in Case 1 of Theorem A and the contraction of all (-1)-hypersurfaces, this needs to be fixed.
- [Section 3, Theorem 3.1] The proof of Theorem 3.1 argues that if f_k(Gamma_k) is not equal to Z_r for all k, then a very general point avoids the union of the f_k(Gamma_k). This is not justified, because f_k(Gamma_k) is only constructible and may be a proper dense subset of Z_r; such a set is not contained in a countable union of proper closed subvarieties. The argument should instead compare dimensions: if all f_k(Gamma_k) have dimension strictly less than nr, their closures are proper closed subsets and a very general point avoids them; otherwise some k has dim f_k(Gamma_k) = nr, which gives dim Gamma_k at least nr as used later. This is a standard repair, but as written the main dimension-counting lemma has a logical gap.
- [Section 3, Corollary 3.2] Corollary 3.2 asserts that r'_0 = h^0(X,T_X) + 1 works because r'_0 is at least dim Aut^0(Y) + 1, where Y is the blow-up of X at m fixed points. The proof does not establish the needed inequality h^0(Y,T_Y) <= h^0(X,T_X). This inequality is true: pushing forward the tangent sheaf of Y gives an injection pi_* T_Y into T_X, so global sections inject. Since Corollary 3.2 is used in the proof of Proposition 4.6, the missing justification should be supplied explicitly.
- [Section 4, Proposition 4.4] Proposition 4.4 as stated fixes d+r points, with the last r points general, but the proof is indexed as though the total number of points were r. Specifically, the proof derives deg L = r/2 and uses the inequality r-d <= dim im(kappa_{L,s}), which corresponds to a total of r points and r-d general points. With the stated d+r points one would have deg L = (d+r)/2 and the number of general coordinates is r, giving r <= (d+r)/2 + c_0, i.e. r <= d + 2c_0, which is still contradicted by the chosen r_0. The statement and proof can be reconciled, and the product 'd < i <= r' in the statement should read 'd < i <= d+r', but as written the proof does not prove the stated proposition. Since Proposition 4.4 is the key input for the ruled-surface case in Proposition 4.6, this is load-bearing.
- [Section 4, Proposition 4.4, parity] The proof of Proposition 4.4 uses a line bundle L with L^2 = O_C(sum z_i), which forces 2 deg L to equal the number of blown-up points; hence the number of points must be even for the argument to run as written. If the total number of points is odd, the determinant computation shows that the assumed involution moving all exceptional divisors cannot exist, so the proposition is still true, but the proof should state this parity discussion explicitly.
minor comments (4)
- [Section 4, Proposition 4.4, statement] The displayed product in the statement of Proposition 4.4 should be over d < i <= d+r, not d < i <= r; as written, the number of factors does not match the indexing of p_{d+1},...,p_{d+r}.
- [Section 4, Proposition 4.6] In the proof of Proposition 4.6, '1 <= j <= p\'' should almost certainly be '1 <= j <= r\''; also 'by Lemma 3.1' should refer to Theorem 3.1 or Corollary 3.2, depending on intent. The indexing 'r_0(X_0,s)' in the definition of A should probably be 'r_0(X_0,i)' for 0 <= i <= s.
- [Section 4, Lemma 4.2] In the proof of Lemma 4.2, the expression 'dim_eta Z_{L,s}' near the end is a typo for 'dim Z_{L,s}'.
- [Section 5, Case 3] In Case 3, the sentence explaining why an automorphism descends to the base curve should explicitly say that any morphism from a rational curve to a curve of positive genus is constant; this is implicit but worth stating for readability.
Circularity Check
No significant circularity: central claim derives from independent dimension-counting and standard external theorems.
full rationale
The paper's main result, Theorem A, is derived from Theorem 3.1, which is proved directly by a dimension-counting argument on the automorphism group scheme: it bounds dim W_{S,\eta,\phi} using the inequality r <= h^0(X,T_X) for n >= 2 and r <= 2h^0(X,T_X) for n = 1. This proof does not assume the conclusion of Theorem A and contains no fitted parameter or normalization that encodes the target result. The remaining cases use standard external facts: the contraction theorem for (-1)-hypersurfaces, [18, Theorem 3.52(2)] for non-uniruled surfaces, and elementary transformations via [23]. The only self-citations, [2] and [3], appear in the introduction as background references and are not load-bearing for the proof. Although Proposition 4.4's proof has an apparent indexing mismatch that may affect its correctness in the ruled-surface case, this is a mathematical gap rather than a circularity: it does not reduce the conclusion to the hypothesis. No step was found where a prediction is equivalent to an input by construction or where a load-bearing premise is justified solely by a self-citation.
Assumptions & free parameters
assumptions (5)
- standard math Aut(X) is a group scheme locally of finite type over C, so Aut(X) minus the identity has countably many connected components of dimension h^0(X,T_X).
- standard math Existence of a contraction of all (-1)-hypersurfaces on a normal projective variety of dimension at least 3, as stated in Lemma 2.2 and citing [18, Theorem 3.7(3)] and [1, Theorem 3.1].
- standard math The birational map from the blown-up surface to its minimal model descends to an isomorphism of a minimal model, via [18, Theorem 3.52(2)].
- standard math A uniruled nonrational surface admits a ruled surface structure over a nonrational curve, and can be written as a blow-up of a minimal ruled surface at points in distinct fibres.
- standard math Elementary transformation lemma: Elm(P(E),p) is isomorphic to (P(F),q) with the expected compatibility of birational maps, as stated in Lemma 2.4 and proved in [23].
Cite this review
Pith. "Pith review of Automorphisms of very general blow up." pith.science (2026). https://pith.science/paper/UX73KMNC
@misc{pith2026260807983,
author = {Pith},
title = {Pith review of: Automorphisms of very general blow up},
year = {2026},
howpublished = {\url{https://pith.science/paper/UX73KMNC}},
note = {Machine review of arXiv:2608.07983}
}
abstract
We show that if $X$ is a projective variety of dimension $\geq 2$ that is not a rational surface, then the blow up of $X$ at sufficiently many very general points has no nontrivial automorphism. Similar results were known before for $\mathbb{P}^2$ and $\mathbb{P}^3.$
Reference graph
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