On sections of Lefschetz fibrations and bundles over 2-complexes

Abstract: We address the question of existence of sections of fibrations in two settings. First, we show that a bundle with base a finite 2-complex admits a section if and only if the inclusion of the fiber is $π_1$-injective and the associated short exact sequence of fundamental groups splits. Second, for Lefschetz fibrations over the disk we provide a complete algebraic criterion characterizing which loops in the boundary mapping torus extend to continuous or smooth sections over the disk. Finally, we apply our results to achiral Lefschetz fibrations over the sphere obtained by doubling along the vertical boundary, and give a criterion ensuring the existence of at least two homologically distinct sections.

• The paper proves that a bundle over a finite 2-complex has a section exactly when the fibre injects on fundamental groups and the fundamental-group sequence splits.
• The paper characterizes Lefschetz-fibration sections through twisted-conjugacy decompositions of vanishing cycles, with smoothability precisely when every multiplicity is 0 or 1.
• The paper applies these criteria to doubles of achiral Lefschetz fibrations, producing pairs of homologically distinct sections when a vanishing cycle survives the monodromy quotient.

This paper by Hillman and Pedrotti addresses the existence of sections of fibrations in two settings: fibre bundles over finite 2-complexes, and Lefschetz fibrations over the disk. The first setting admits a purely algebraic characterization via fundamental groups; the second requires genuinely geometric input from the monodromy factorization. As an application, the authors derive a criterion guaranteeing homologically distinct sections of achiral Lefschetz fibrations over $S^2$ obtained by doubling along vertical boundary.

Sections of bundles over 2-complexes

The first main result gives a complete homotopy-theoretic criterion for sections of a fibre bundle $p: E \to B$ with base a finite 2-complex (2604.10943). A section exists if and only if two conditions hold simultaneously:

• Fibre injectivity: the inclusion $F = p^{-1}(\ast_B) \hookrightarrow E$ induces a monomorphism $\pi_1 F \to \pi_1 E$;
• Splitting: there is a homomorphism $\mathfrak{s}: \pi_1 B \to \pi_1 E$ with $p_* \mathfrak{s} = \mathrm{Id}$.

Moreover, any such splitting homomorphism can be realized as the induced map of an actual section. Necessity follows from the long exact sequence of homotopy; sufficiency is proved by induction over the 2-cells of $B$, using the Homotopy Lifting Property to extend sections over the 1-skeleton and then across each 2-cell via a Van Kampen argument showing that the boundary restriction is null-homotopic in the trivialized piece $F \times D$. An alternative proof for aspherical bases uses the pullback bundle $\chi^*E$ over the characteristic map together with the Homotopy Extension Lifting Property.

The injectivity hypothesis is automatic when $B$ is aspherical or when $p: E \to B$0, since the image of $p: E \to B$1 in $p: E \to B$2 under the connecting homomorphism is central (Gottlieb). The Hopf fibration composed with a projection provides a torus-bundle over $p: E \to B$3 with no section, showing both hypotheses are needed. A flat 4-manifold group example demonstrates that section existence is a property of the bundle map, not of the total space alone: the same group arises both as a Klein-bottle bundle without a section and as a torus bundle with one.

The paper also notes that for bases of dimension greater than 2, fundamental group considerations are insufficient — the quaternionic Hopf fibration $p: E \to B$4 has no section because $p: E \to B$5 — and poses open questions about constructing bundles over $p: E \to B$6 with compact manifold fibre (e.g., $p: E \to B$7) lacking sections, including a candidate $p: E \to B$8-bundle built from Hatcher's computation of $p: E \to B$9.

Lefschetz fibrations over the disk

For Lefschetz fibrations, homotopy theory alone does not determine section existence; the obstruction is geometric and tied to vanishing cycles. The strategy follows the standard decomposition of the base sphere into a critical disk containing all critical values and a trivial disk, reducing the problem to characterizing which boundary loops extend as sections over the critical disk.

The technical foundation is Seidel's model of a symplectic Lefschetz fibration $F = p^{-1}(\ast_B) \hookrightarrow E$0 over the disk with a single critical point and prescribed vanishing cycle $F = p^{-1}(\ast_B) \hookrightarrow E$1, whose boundary is identified with the mapping torus $F = p^{-1}(\ast_B) \hookrightarrow E$2 of the Dehn twist along $F = p^{-1}(\ast_B) \hookrightarrow E$3. Two families of boundary sections play a central role: $F = p^{-1}(\ast_B) \hookrightarrow E$4 (wrapping once around a based representative $F = p^{-1}(\ast_B) \hookrightarrow E$5 at the "top" of the cotangent annulus) and $F = p^{-1}(\ast_B) \hookrightarrow E$6 (a half-twisted variant), connected by an explicit free homotopy through sections, with multiplicity-$F = p^{-1}(\ast_B) \hookrightarrow E$7 analogues $F = p^{-1}(\ast_B) \hookrightarrow E$8 and $F = p^{-1}(\ast_B) \hookrightarrow E$9.

Two structural lemmas organize the analysis. First, sections of a mapping torus $\pi_1 F \to \pi_1 E$0 correspond bijectively to twisted loops in $\pi_1 F \to \pi_1 E$1, so every class $\pi_1 F \to \pi_1 E$2 in $\pi_1 F \to \pi_1 E$3 is realized by some smooth section whenever the basepoint lies in $\pi_1 F \to \pi_1 E$4. Second, two based sections representing $\pi_1 F \to \pi_1 E$5 and $\pi_1 F \to \pi_1 E$6 are homotopic through sections if and only if $\pi_1 F \to \pi_1 E$7 — i.e., they differ by twisted conjugation.

Single critical value

For the fibration with one vanishing cycle $\pi_1 F \to \pi_1 E$8, the paper proves that a boundary loop $\pi_1 F \to \pi_1 E$9 extends to a continuous section passing through the critical point if and only if

$\mathfrak{s}: \pi_1 B \to \pi_1 E$0

for some loop $\mathfrak{s}: \pi_1 B \to \pi_1 E$1 and integer $\mathfrak{s}: \pi_1 B \to \pi_1 E$2. The section can be taken to avoid the critical point — hence be smoothable — if and only if $\mathfrak{s}: \pi_1 B \to \pi_1 E$3. The forward direction for smoothable sections proceeds by smoothing relative to the boundary, exploiting that away from the thimble the fibration is trivial; the continuous case analyzes the section's behavior inside the local model $\mathfrak{s}: \pi_1 B \to \pi_1 E$4, where the relevant mapping torus of the Dehn twist restricted to a neighborhood of $\mathfrak{s}: \pi_1 B \to \pi_1 E$5 is shown via an explicit coordinate change to be the trivial bundle, forcing the boundary class into the $\mathfrak{s}: \pi_1 B \to \pi_1 E$6 family. This result is a converse to Seidel's classification of pseudo-holomorphic thimbles with Lagrangian boundary condition, which realize precisely the classes $\mathfrak{s}: \pi_1 B \to \pi_1 E$7 and $\mathfrak{s}: \pi_1 B \to \pi_1 E$8.

Multiple critical values

The general criterion combines these ingredients. Given a positive factorization $\mathfrak{s}: \pi_1 B \to \pi_1 E$9 corresponding to a geometric basis of $p_* \mathfrak{s} = \mathrm{Id}$0, the key algebraic tool is an injective map

$p_* \mathfrak{s} = \mathrm{Id}$1

sending the generator of $p_* \mathfrak{s} = \mathrm{Id}$2 to the product word $p_* \mathfrak{s} = \mathrm{Id}$3 in the free group. Injectivity holds because this word has infinite order in $p_* \mathfrak{s} = \mathrm{Id}$4. Concatenating per-thimble boundary data then yields the main theorem: a boundary loop $p_* \mathfrak{s} = \mathrm{Id}$5 extends to a continuous section over the disk if and only if $p_* \mathfrak{s} = \mathrm{Id}$6 decomposes as

$p_* \mathfrak{s} = \mathrm{Id}$7

where each $p_* \mathfrak{s} = \mathrm{Id}$8 is twisted-conjugate to a multiple $p_* \mathfrak{s} = \mathrm{Id}$9 of the $B$0-th vanishing cycle. Smoothability holds exactly when all $B$1. The proof reduces to the bouquet of disks via the observation that sections over $B$2 agree, up to homotopy through sections, with sections over the bouquet, since the complement is contractible.

A notable point is well-definedness: although the factorization of $B$3 is auxiliary, extendability of $B$4 is invariant under Hurwitz moves on the factorization. The paper verifies this explicitly for $B$5, showing that the two descriptions of the same class under the Hurwitz-transformed basis produce compatible decompositions. Thus extendability is intrinsic to the loop and the fibration, not to the choice of vanishing paths.

Doubles and homologically distinct sections

The final application concerns achiral Lefschetz fibrations $B$6 obtained by doubling a Lefschetz fibration $B$7 with closed fiber along its vertical boundary. In contrast to Gompf's construction of achiral Lefschetz fibrations over $B$8 with arbitrarily high genus fiber and no sections — where non-primitivity of $B$9 in $F \times D$0 obstructs sections — doubling here introduces no homological obstruction, and many sections exist: each vector $F \times D$1 yields a section $F \times D$2 of $F \times D$3 whose double $F \times D$4 sections $F \times D$5.

The criterion for homological distinctness uses the Mayer–Vietoris boundary map

$F \times D$6

If some vanishing cycle satisfies $F \times D$7, then $F \times D$8 while the Picard–Lefschetz formula gives $F \times D$9, so the two doubles are homologically distinct. When $\chi^*E$0 is isotopic to the identity, the image of $\chi^*E$1 vanishes and Smith's results guarantee a homologically essential vanishing cycle, yielding countably many homologically distinct sections of $\chi^*E$2.

Limitations and open questions

Several caveats qualify the results. The bundle criterion applies only to bases of dimension at most 2; the authors themselves exhibit higher-dimensional obstructions ($\chi^*E$3, weakly contractible structure groups) lying outside fundamental-group analysis, and leave open whether their construction of a $\chi^*E$4-bundle over $\chi^*E$5 with no section can be modified to have compact manifold fibre such as $\chi^*E$6. Whether Hatcher-inspired clutching function $\chi^*E$7 classifies a generator of $\chi^*E$8, and whether that bundle sections, remain unanswered. On the Lefschetz side, the criterion resolves extension over the critical disk but does not settle the standing problem (BKR26, Problem 2.22) of whether every closed Lefschetz fibration over $\chi^*E$9 admits a section; the doubling construction produces achiral fibrations with many sections rather than addressing chiral cases. Finally, the smoothability condition $B$0 is stated for sections extending given boundary data; the interaction between smoothability and global holomorphic or symplectic constraints is not pursued.

Conclusion

The paper delivers a clean dichotomy: for bundles over finite 2-complexes, section existence is exactly the conjunction of $B$1-injectivity of the fibre inclusion and splittability of the fundamental group exact sequence; for Lefschetz fibrations over the disk, it is exactly a twisted-conjugacy decomposition of the boundary loop against multiples of the vanishing cycles, with smoothability equivalent to all multiplicities being 0 or 1. The Hurwitz-invariance of the criterion makes it intrinsic to the fibration, and the doubling application converts it into a concrete sufficient condition for pairs of homologically distinct sections of achiral Lefschetz fibrations over the sphere.