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Is there a topological Bogomolov--Miyaoka--Yau inequality?

Algebraic Geometry · Submitted 24 Feb 2006 · 78 references

Pith's verdict: A useful conjecture-and-examples note; the Section 5 examples are credible, but the w*=1 density claim needs a cleaner proof.

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arXiv IDmath/0602562 (version 2)
Pith ID6KZ42BJU, Pith's permanent record of this paper and its review
CitationJ\'anos Koll\'ar (Princeton Univ). "Is there a topological Bogomolov--Miyaoka--Yau inequality?." arXiv:math/0602562 (2006). https://pith.science/paper/6KZ42BJU
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Referee report
The paper

The paper claims

This paper proposes a topological form of the Bogomolov–Miyaoka–Yau inequality for smooth four-manifolds with spherical boundary, and builds algebraic surfaces that come close to the limit.

The reading

The paper asks whether the Bogomolov–Miyaoka–Yau inequality, which for normal projective surfaces with quotient singularities reads $\sum_{s\in\operatorname{Sing} S}(1-1/|\pi_1(L_s)|)\le 3$, is actually a topological statement about smooth compact 4-manifolds whose boundary components are spherical. The proposed Conjecture 2 asserts that for any such $M$ with $H_1(M,\mathbb Z)=0$ and $H_2(M,\mathbb Z)\cong\mathbb Z$, the same sum over the boundary components is $<3$. The paper shows that a proof of this inequality would nearly settle the Montgomery–Yang problem on circle actions on the 5-sphere and would constrain which Seifert-fibered 3-manifolds bound a 4-dimensional homology ball. On the algebraic side it constructs, inside a family of weighted projective hypersurfaces $x_1^{a_1}x_2+\cdots+x_4^{a_4}x_1=0$, rational surfaces with quotient singularities that have the rational homology of $\mathbb{CP}^2$, ample canonical class, and $(K_{S^*})^2$ approaching 1 as the exponents grow.

What carries the argument

The machinery has four pieces. First, the orbifold Euler characteristic $e_{\mathrm{orb}}(S)=e(S)-\sum_s(1-1/|\pi_1(L_s)|)$ turns the algebraic Bogomolov–Miyaoka–Yau inequality into the boundary sum (1.3), and the topological conjecture is the assertion that this sum is a smooth invariant bounded by 3. Second, the correspondence of Theorem 8 between pseudo-free circle actions on rational homology 5-spheres and 4-manifolds with lens-space boundary converts that bound into a statement about nonfree orbit stabilizers. Third, the family $H(a_1,\ldots,a_n)\subset\mathbb P(w_1,\ldots,w_n)$ defined by $\sum x_i^{a_i}x_{i+1}=0$ has weights determined by the linear system $a_iw_i+w_{i+1}=d$, and the ratio condition $w^*=\gcd(W_1,\ldots,W_n)=1$ makes the linear system $|H|$ a birational map to $\mathbb{CP}^{n-2}$ via the torus homomorphism $\eta^*y_i=x_i^{a_i}x_{i+1}$. Fourth, the Milnor–Orlik formula computes the rational homology of these hypersurfaces, and the adjunction formula for log divisors allows the contraction of the two disjoint rational curves in the $n=4$ case.

Extended reading notes

Core claim

The central claim is Conjecture 2: no smooth compact 4-manifold with $H_1(M,\mathbb Z)=0$ and $H_2(M,\mathbb Z)\cong\mathbb Z$ can have spherical boundary components $L_i$ with $\sum_i(1-1/|\pi_1(L_i)|)\ge 3$; in particular such an $M$ has at most five boundary components. The author does not prove this conjecture, but shows it would imply the Montgomery–Yang bound that pseudo-free circle actions on rational homology 5-spheres have at most four nonfree orbits, and he gives a short list of possible exceptional stabilizer patterns. The proved contribution is a family of examples: for exponent tuples with the numerical condition $w^*=1$, the hypersurface $S(a_1,a_2,a_3,a_4)$ is rational, has four quotient singularities, and after contracting two disjoint rational curves becomes $S^*(a_1,\ldots,a_4)$, a rational surface with quotient singularities, rational homology of $\mathbb{CP}^2$, ample canonical class when all $a_i\ge 4$, and $K_{S^*}^2\to 1$. These are, in the paper's view, illustrations that the topological inequality sits at the edge of what algebraic surfaces can do.

Load-bearing premise

The abundance of examples rests on the numerical gcd condition $w^*=1$, which the paper cannot characterize; the supporting claim that this condition, and the similar gcd in Example 27, holds for at least 75 percent of exponent tuples is asserted without proof.

Editorial extensions

If this is right

  • A proof of Conjecture 2 would nearly settle the Montgomery–Yang problem, reducing it to checking four explicit stabilizer patterns: $k\le 3$ nonfree orbits, $(2,3,5,n)$ with $(n,30)=1$, $(2,3,7,n)$ for $n$ in a short list, or $(2,3,11,13)$.
  • For Seifert fibered rational homology spheres that are rationally H-cobordant to zero, the same bound would force $\sum(1-1/d_i)<3$; for integral homology spheres $\Sigma(d_1,\ldots,d_k)$ H-cobordant to zero, it would force $k\le 3$.
  • The constructed surfaces $S^*(a_1,\ldots,a_4)$ would be rational homology $\mathbb{CP}^2$ surfaces with ample canonical class and $K^2$ arbitrarily close to 1, showing that the algebraic inequality (1.3) can be nearly saturated with four quotient singularities.
  • The bound would be genuinely smooth-topological: with topological homology cells, one can build homology $\mathbb{CP}^2$"s with arbitrarily many singular points, so the smooth hypothesis is essential.
  • If the $w^*=1$ condition holds, the example family supplies infinitely many rational surfaces that are birational to $\mathbb{CP}^2$ yet have ample canonical class, separating birational type from Kodaira dimension in the singular setting.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The $w^*=1$ condition is a concrete number-theoretic obstruction, so a finite computer search over exponent tuples could determine its actual density and test the paper's asymptotic 0.75 heuristic.
  • If Conjecture 2 is true, the boundary defect $\sum(1-1/|\pi_1(L_i)|)$ would function as a new smooth invariant of 4-manifolds with spherical boundary, and one could ask how it behaves under surgery and connected sums.
  • The paper's suggested generalization (2.2) to arbitrary $H_2(M,\mathbb Q)$ could first be tested on algebraic surfaces with larger Picard number, such as the Hacking examples, before being taken as a topological principle.
  • Applying the same $w^*$ machinery to the $n=3$ case may yield new Seifert fibered rational homology spheres that are rationally H-cobordant to zero, extending the link between algebraic hypersurfaces and the H-cobordism problem.
Original abstract

The aim of this paper is to consider a possible extension of the Bogomolov--Miyaoka--Yau inequality to differentiable orbifolds. The conjectured extension is related to the Montgomery--Yang problem about circle actions on the 5--sphere and also to the H--cobordism of Seifert fibered 3--manifolds. Related conjectures on algebraic surfaces with quotient singularities which have the same rational homology as the projective plane are also considered. Finally we give such examples which are birational to the projective plane yet have ample canonical class.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 3,637 input tokens.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.17 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.06 · compares with prior results 0.18 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

The aim of this paper is to consider a possible extension of the Bogomolov--Miyaoka--Yau inequality to differentiable orbifolds. The conjectured extension is related to the Montgomery--Yang problem about circle actions on the 5--sphere and also to the H--cobordism of Seifert fibered 3--manifolds. Related conjectures on algebraic surfaces with quotient singularities which have the same rational homology as the projective plane are also considered. Finally we give such examples which are birational to the projective plane yet have ample canonical class.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 The aim of this paper is to consider a possible extension of the Bogomolov--Miyaoka--Yau inequality to differentiable orbifolds. contribution 0.76 0.39 0.05 hedged possibility 0.63
s1 The conjectured extension is related to the Montgomery--Yang problem about circle actions on the 5--sphere and also to the H--cobordism of Seifert fibered 3--manifolds. background 0.76 0.14 0.04 asserted 0.71
s2 Related conjectures on algebraic surfaces with quotient singularities which have the same rational homology as the projective plane are also considered. contribution 0.75 0.51 0.04 asserted 0.80
s3 Finally we give such examples which are birational to the projective plane yet have ample canonical class. result 0.65 0.91 0.35 asserted 1.00

What kind of paper the classifier reports this is: theoretical result 0.51 · evidence from proof or derivation 0.55.

What was examined, and what was not

sources read
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  • figures and tables
  • references and citing sentences
  • relations to other papers
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  • P abstract
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  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References
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Discussion

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