Holomorphically parallelizable solvmanifolds with special metrics and their deformations
Pith reviewed 2026-05-07 13:11 UTC · model grok-4.3
The pith
Deformations of the holomorphically parallelizable Nakamura manifold with non-left-invariant complex structures admit balanced metrics but no strong Kähler with torsion metrics.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper shows that the class of deformations of the holomorphically parallelizable Nakamura manifold yielding a non-left-invariant complex structure admits a balanced metric but does not admit any strong Kähler with torsion metric. It investigates the existence of strong Kähler with torsion metrics along deformations of the Iwasawa manifold and of the holomorphically parallelizable Nakamura manifold, constructs the Kuranishi space of a 4-dimensional holomorphically parallelizable solvmanifold to study whether small deformations admit SKT metrics, and provides results on the existence of metrics satisfying partial bar partial omega equals zero and partial bar partial omega squared equals 0.
What carries the argument
The Kuranishi space of infinitesimal deformations of the complex structure on holomorphically parallelizable solvmanifolds, combined with explicit checks of the torsion condition partial bar partial omega equals zero that defines strong Kähler with torsion metrics.
If this is right
- Deformations of the Iwasawa manifold admit SKT metrics only in certain directions within the deformation space.
- Non-left-invariant deformations of the Nakamura manifold consistently support balanced metrics.
- Small deformations inside the Kuranishi space of the four-dimensional solvmanifold admit SKT metrics only for some parameters.
- On the considered two-step nilpotent nilmanifolds, metrics exist that simultaneously satisfy partial bar partial omega equals zero and partial bar partial omega squared equals zero.
Where Pith is reading between the lines
- The loss of left-invariance under deformation appears to be the mechanism that eliminates SKT metrics while preserving balanced ones.
- The same deformation analysis could be applied to other classes of solvmanifolds to test whether the balanced-but-not-SKT pattern is general.
- Explicit coordinate computations on low-dimensional examples would provide concrete test cases for the metric conditions.
Load-bearing premise
The solvmanifolds are holomorphically parallelizable so that their deformations and the resulting Hermitian metrics can be described explicitly using standard deformation theory.
What would settle it
An explicit construction of a non-left-invariant deformation of the Nakamura manifold together with a Hermitian metric omega satisfying partial bar partial omega equals zero would show that the non-existence claim for SKT metrics does not hold in that class.
read the original abstract
We investigate the existence of strong K\"ahler with torsion metrics along deformations of the Iwasawa manifold and of the holomorphically parallelizable Nakamura manifold. We also show that the class of deformations of the holomorphically parallelizable Nakamura manifold yielding a non-left-invariant complex structure admits a balanced metric but does not admit any strong K\"ahler with torsion metric. We then construct the Kuranishi space of a $4$-dimensional holomorphically parallelizable solvmanifold and study whether small deformations of such a manifold admit SKT metrics. Finally, we provide some results concerning the existence of metrics satisfying $\partial \bar{\partial} \omega = 0$, $\partial \bar{\partial} \omega^2 = 0$ on a particular class of $2$-step nilpotent nilmanifolds.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper investigates the existence of strong Kähler with torsion (SKT) metrics along deformations of the Iwasawa manifold and the holomorphically parallelizable Nakamura manifold. It shows that deformations of the Nakamura manifold yielding non-left-invariant complex structures admit balanced metrics but no SKT metrics. The Kuranishi space of a 4-dimensional holomorphically parallelizable solvmanifold is constructed and small deformations are studied for the existence of SKT metrics. Results are also given on the existence of metrics satisfying ∂∂̄ω = 0 and ∂∂̄ω² = 0 for a class of 2-step nilpotent nilmanifolds.
Significance. If the results are correct, the paper supplies concrete examples distinguishing the existence of balanced metrics from the non-existence of SKT metrics on solvmanifolds with non-left-invariant complex structures. The explicit deformation analysis and Kuranishi space construction add to the deformation theory of special Hermitian metrics on homogeneous non-Kähler manifolds.
major comments (2)
- [Deformations of the holomorphically parallelizable Nakamura manifold] The central non-existence claim for SKT metrics on the non-left-invariant deformations of the Nakamura manifold (stated in the abstract and developed in the corresponding section) requires a global obstruction applying to arbitrary Hermitian metrics. The argument must be checked to ensure it does not rely on averaging or Nomizu-type reductions that presuppose left-invariance of the complex structure J, as such reductions fail once J is deformed to be non-left-invariant.
- [Kuranishi space construction] In the section constructing the Kuranishi space of the 4-dimensional solvmanifold, the criteria used to determine which small deformations admit SKT metrics should be stated explicitly (e.g., via the relevant cohomology classes or the ∂∂̄-lemma failure), together with the dimension of the space and the parameter range considered.
minor comments (2)
- [Abstract] The abstract refers to 'a particular class of 2-step nilpotent nilmanifolds' without further identification; adding a brief characterizing property or reference would aid readability.
- [Notation and preliminaries] Notation for the Hermitian form ω and the complex structure should be introduced uniformly before the deformation sections to avoid ambiguity when moving between invariant and non-invariant cases.
Simulated Author's Rebuttal
We thank the referee for the careful reading of our manuscript and the constructive comments. We address the two major comments point by point below. Where clarification is needed, we will revise the text accordingly in the next version.
read point-by-point responses
-
Referee: The central non-existence claim for SKT metrics on the non-left-invariant deformations of the Nakamura manifold (stated in the abstract and developed in the corresponding section) requires a global obstruction applying to arbitrary Hermitian metrics. The argument must be checked to ensure it does not rely on averaging or Nomizu-type reductions that presuppose left-invariance of the complex structure J, as such reductions fail once J is deformed to be non-left-invariant.
Authors: The non-existence of SKT metrics on these deformations is established via a global cohomological obstruction that holds for any Hermitian metric and does not invoke averaging over the group or any Nomizu-type theorem. The argument relies instead on the holomorphic parallelizability of the deformed manifold together with the explicit failure of the ∂∂̄-lemma on the underlying solvmanifold, which persists under the deformation to non-left-invariant complex structures. We will insert a short clarifying paragraph immediately after the statement of the theorem to emphasize that the obstruction is intrinsic and independent of left-invariance. revision: yes
-
Referee: In the section constructing the Kuranishi space of the 4-dimensional solvmanifold, the criteria used to determine which small deformations admit SKT metrics should be stated explicitly (e.g., via the relevant cohomology classes or the ∂∂̄-lemma failure), together with the dimension of the space and the parameter range considered.
Authors: We agree that the criteria and the geometric data of the Kuranishi space should be stated more explicitly. In the revised manuscript we will add a dedicated paragraph listing the precise conditions (vanishing of a certain class in H^{1,1} and the explicit failure of the ∂∂̄-lemma) that characterize the SKT locus. The Kuranishi space is two-dimensional, and we consider deformations inside a sufficiently small ball in the parameter space; these details will be recorded together with the explicit equations defining the SKT subset. revision: yes
Circularity Check
No circularity: standard deformation theory and explicit constructions without self-referential reductions
full rationale
The paper's claims rest on direct application of Kuranishi deformation theory, explicit metric constructions for balanced metrics, and standard obstructions (such as failure of the ∂∂̄-lemma or non-vanishing classes) for non-existence of SKT metrics. These are applied to deformations of holomorphically parallelizable solvmanifolds, including non-left-invariant cases on the Nakamura manifold. No equations or arguments reduce by construction to fitted parameters renamed as predictions, self-definitions, or load-bearing self-citations whose validity depends on the current paper. The central non-existence result for SKT metrics is presented as following from global obstructions independent of left-invariance, and balanced-metric existence is shown constructively. This is self-contained against external benchmarks in complex geometry, consistent with the reader's assessment of no circularity indicators in the abstract or setup.
Axiom & Free-Parameter Ledger
Reference graph
Works this paper leans on
-
[1]
E. Abbena and A. Grassi,Hermitian left invariant metrics on complex Lie groups and cosymplectic Hermitian mani- folds, Boll. Un. Mat. Ital. A (6)5(1986), no. 3, 371–379.↑2
work page 1986
-
[2]
Alessandrini,Proper modifications of generalizedp-K¨ ahler manifolds, J
L. Alessandrini,Proper modifications of generalizedp-K¨ ahler manifolds, J. Geom. Anal.27(2017), no. 2, 947–967.↑4
work page 2017
-
[3]
L. Alessandrini and G. Bassanelli,Small deformations of a class of compact non-K¨ ahler manifolds, Proc. Amer. Math. Soc.109(1990), no. 4, 1059–1062.↑3 HOLOMORPHICALLY PARALLELIZABLE SOLVMANIFOLDS, SPECIAL METRICS AND DEFORMATIONS 21
work page 1990
-
[4]
,Smooth proper modifications of compact K¨ ahler manifolds, Complex analysis (Wuppertal, 1991), 1991, pp. 1– 7.↑2
work page 1991
-
[5]
B. Alexandrov and S. Ivanov,Vanishing theorems on Hermitian manifolds, Differential Geom. Appl.14(2001), no. 3, 251–265.↑2
work page 2001
-
[6]
D. Angella,The cohomologies of the Iwasawa manifold and of its small deformations, The Journal of Geometric Analysis23(December 2013), 1355–1378.↑3
work page 2013
-
[7]
D. Angella, M. G. Franzini, and F. A. Rossi,Degree of non-K¨ ahlerianity for 6-dimensional nilmanifolds, Manuscripta Math.148(2015), no. 1-2, 177–211.↑3
work page 2015
-
[8]
D. Angella, V. Guedj, and C. H. Lu,Plurisigned Hermitian metrics, Trans. Amer. Math. Soc.376(2023), no. 7, 4631–4659.↑2
work page 2023
-
[9]
D. Angella and A. Tomassini,On cohomological decomposition of almost-complex manifolds and deformations, Journal of Symplectic Geometry9(2011), 403–428.↑3, 11
work page 2011
-
[10]
,On the∂ ∂-lemma and Bott-Chern cohomology, Invent. Math.192(2013), no. 1, 71–81.↑3
work page 2013
-
[11]
D. Angella and L. Ugarte,On small deformations of balanced manifolds, Differential Geom. Appl.54(2017), 464–474. ↑3
work page 2017
-
[12]
Bismut,A local index theorem for non-K¨ ahler manifolds, Math
J.-M. Bismut,A local index theorem for non-K¨ ahler manifolds, Math. Ann.284(1989), no. 4, 681–699.↑1
work page 1989
-
[13]
Ciulic˘ a,A new class of non-K¨ ahler metrics, Complex Manifolds12(2025), no
C. Ciulic˘ a,A new class of non-K¨ ahler metrics, Complex Manifolds12(2025), no. 1, Paper No. 20250014, 14.↑3, 8
work page 2025
-
[14]
C. Ciulic˘ a, A. Otiman, and M. Stanciu,Special non-K¨ ahler metrics on Endo-Pajitnov manifolds, Ann. Mat. Pura Appl. (4)204(2025), no. 4, 1425–1441.↑3
work page 2025
-
[15]
L. Cordero, M. Fern´ andez, A. Gray, and L. Ugarte,Nilpotent complex structures on compact nilmanifolds, Rend. Circ. Mat. Palermo (2) Suppl.49(1997), 83–100.↑6
work page 1997
-
[16]
L. A. Cordero, M. Fern´ andez, A. Gray, and L. Ugarte,Compact nilmanifolds with nilpotent complex structures: Dol- beault cohomology, Trans. Amer. Math. Soc.352(2000), no. 12, 5405–5433.↑6
work page 2000
-
[17]
Ehresmann,Sur les espaces fibr´ es diff´ erentiables, C
C. Ehresmann,Sur les espaces fibr´ es diff´ erentiables, C. R. Acad. Sci. Paris224(1947), 1611–1612.↑5
work page 1947
-
[18]
A. Fino, G. Grantcharov, and L. Vezzoni,Astheno-K¨ ahler and balanced structures on fibrations, Int. Math. Res. Not. IMRN22(2019).↑2
work page 2019
-
[19]
A. Fino, H. Kasuya, and L. Vezzoni,SKT and tamed symplectic structures on solvmanifolds, Tohoku Math. J. (2)67 (2015), no. 1, 19–37.↑2
work page 2015
-
[20]
A. Fino, M. Parton, and S. Salamon,Families of strong KT structures in six dimensions, Comment. Math. Helv.79 (2004), no. 2, 317–340.↑3, 7
work page 2004
-
[21]
A. Fino and A. Tomassini,Blow-ups and resolutions of strong K¨ ahler with torsion metrics, Adv. Math.221(2009), no. 3, 914–935.↑2, 3, 11
work page 2009
-
[22]
,On astheno-K¨ ahler metrics, J. Lond. Math. Soc. (2)83(2011), no. 2, 290–308.↑2, 3, 4, 8
work page 2011
-
[23]
A. Fino and L. Vezzoni,Special Hermitian metrics on compact solvmanifolds, J. Geom. Phys.91(2015), 40–53.↑2
work page 2015
-
[24]
,On the existence of balanced and SKT metrics on nilmanifolds, Proc. Amer. Math. Soc.144(2016), no. 6, 2455–2459.↑2, 12
work page 2016
-
[25]
M. G. Franzini,Deformazioni di variet` a bilanciate e loro propriet` a coomologiche, 2011. Tesi di Laurea Magistrale, Universit` a di Parma.↑13
work page 2011
-
[26]
Gauduchon,Fibr´ es hermitiens ` a endomorphisme de Ricci non n´ egatif, Bull
P. Gauduchon,Fibr´ es hermitiens ` a endomorphisme de Ricci non n´ egatif, Bull. Soc. Math. France105(1977), no. 2, 113–140.↑1
work page 1977
-
[27]
,La1-forme de torsion d’une vari´ et´ e hermitienne compacte, Math. Ann.267(1984), no. 4, 495–518.↑1
work page 1984
-
[28]
,Hermitian connections and Dirac operators, Boll. Un. Mat. Ital. B (7)11(1997), no. 2, 257–288.↑2
work page 1997
-
[29]
Goto,Deformations of generalized complex and generalized K¨ ahler structures, J
R. Goto,Deformations of generalized complex and generalized K¨ ahler structures, J. Differential Geom.84(2010), no. 3, 525–560.↑3
work page 2010
-
[30]
A. Gray and L. M. Hervella,The sixteen classes of almost Hermitian manifolds and their linear invariants, Ann. Mat. Pura Appl. (4)123(1980), 35–58.↑1
work page 1980
-
[31]
B. Guan and Q. Li,Complex Monge-Amp` ere equations and totally real submanifolds, Adv. Math.225(2010), no. 3, 1185–1223.↑2
work page 2010
-
[32]
R. Harvey and A. W. Knapp,Positive(p, p)forms, Wirtinger’ inequality, and currents, Value distribution theory (Proc. Tulane Univ. Program, Tulane Univ., New Orleans, La., 1972-1973), Part A, 1974, pp. 43–62.↑4
work page 1972
-
[33]
K. Hasegawa,Small deformations and non-left-invariant complex structures on six-dimensional compact solvmanifolds, Differential Geom. Appl.28(2010), no. 2, 220–227.↑14
work page 2010
-
[34]
R. Hind, C. Medori, and A. Tomassini,Families of almost complex structures and transverse(p, p)-forms, J. Geom. Anal.33(2023), no. 10, Paper No. 334, 23.↑12
work page 2023
-
[35]
Huybrechts,Complex Geometry:An Introduction, Universitext, Springer Berlin Heidelberg, 2005.↑5
D. Huybrechts,Complex Geometry:An Introduction, Universitext, Springer Berlin Heidelberg, 2005.↑5
work page 2005
-
[36]
J. Jost and S.-T. Yau,A nonlinear elliptic system for maps from Hermitian to Riemannian manifolds and rigidity theorems in Hermitian geometry, Acta Math.170(1993), no. 2, 221–254.↑1
work page 1993
-
[37]
Kasuya,de Rham and Dolbeault cohomology of solvmanifolds with local systems, Math
H. Kasuya,de Rham and Dolbeault cohomology of solvmanifolds with local systems, Math. Res. Lett.21(2014), no. 4, 781–805.↑15
work page 2014
-
[38]
,Generalized deformations and holomorphic Poisson cohomology of solvmanifolds, Ann. Global Anal. Geom. 51(2017), no. 2, 155–177.↑15 22 ETTORE LO GIUDICE, LAPO RUBINI AND ADRIANO TOMASSINI
work page 2017
-
[39]
K. Kodaira,Complex Manifolds and Deformations of Complex Structures, Classics in Mathematics, Springer Berlin, Heidelberg, 2005.↑3, 5
work page 2005
-
[40]
K. Kodaira and J. Morrow,Complex Manifolds, AMS Chelsea Publishing, 2006.↑5
work page 2006
-
[41]
K. Kodaira and D. C. Spencer,On deformations of complex analytic structures. III. Stability theorems for complex structures, Ann. of Math. (2)71(1960), 43–76.↑3
work page 1960
-
[42]
A. Latorre and L. Ugarte,On non-K¨ ahler compact complex manifolds with balanced and astheno-K¨ ahler metrics, C. R. Math. Acad. Sci. Paris355(2017), no. 1, 90–93.↑2
work page 2017
-
[43]
A. I. Malcev,On a class of homogeneous spaces, Amer. Math. Soc. Translation Ser. 19(1962), 276–307.↑9
work page 1962
-
[44]
K. Matsuo and T. Takahashi,On compact astheno-K¨ ahler manifolds, Colloq. Math.89(2001), no. 2, 213–221.↑2
work page 2001
-
[45]
M. L. Michelsohn,On the existence of special metrics in complex geometry, Acta Math.149(1982), no. 3-4, 261–295. ↑1, 2
work page 1982
-
[46]
Nakamura,Complex parallelisable manifolds and their small deformations, J
I. Nakamura,Complex parallelisable manifolds and their small deformations, J. Differential Geometry10(1975), 85– 112.↑3, 4, 10, 12, 13, 14, 15
work page 1975
-
[47]
R. Piovani and T. Sferruzza,Deformations of Strong K¨ ahler with torsion metrics, Complex Manifolds8(2021), no. 1, 286–301.↑3, 8, 9
work page 2021
-
[48]
D. Popovici,Deformation limits of projective manifolds: Hodge numbers and strongly Gauduchon metrics, Invent. Math.194(2013), no. 3, 515–534.↑3
work page 2013
-
[49]
S. Rao, X. Wan, and Q. Zhao,On local stabilities ofp-K¨ ahler structures, Compos. Math.155(2019), no. 3, 455–483. ↑3
work page 2019
-
[50]
,Power series proofs for local stabilities of K¨ ahler and balanced structures with mild∂ ∂-lemma, Nagoya Math. J.246(2022), 305–354.↑3
work page 2022
- [51]
-
[52]
,Several special complex structures and their deformation properties, J. Geom. Anal.28(2018), no. 4, 2984– 3047.↑4, 5, 6
work page 2018
-
[53]
Sferruzza,Deformations of balanced metrics, Bull
T. Sferruzza,Deformations of balanced metrics, Bull. Sci. Math.178(2022), Paper No. 103143, 23.↑3
work page 2022
-
[54]
,Deformations of astheno-K¨ ahler metrics, Complex Manifolds10(2023), no. 1, Paper No. 20230102, 19.↑3, 4, 8
work page 2023
-
[55]
T. Sferruzza and A. Tomassini,On cohomological and formal properties of strong K¨ ahler with torsion and astheno- K¨ ahler metrics, Math. Z.304(2023), no. 4, Paper No. 55, 27.↑2, 4, 7
work page 2023
-
[56]
,Bott-Chern formality and Massey products on strong K¨ ahler with torsion and K¨ ahler solvmanifolds, J. Geom. Anal.34(2024), no. 11.↑3, 4, 15, 19
work page 2024
-
[57]
H.-C. Wang,Complex parallisable manifolds, Proceedings of the American Mathematical Society5(1954), 771–776. ↑2
work page 1954
-
[58]
Wu,On the geometry of superstrings with torsion, ProQuest LLC, Ann Arbor, MI, 2006
C.-C. Wu,On the geometry of superstrings with torsion, ProQuest LLC, Ann Arbor, MI, 2006. Thesis (Ph.D.)–Harvard University.↑3
work page 2006
-
[59]
Xiao,On strongly Gauduchon metrics of compact complex manifolds, J
J. Xiao,On strongly Gauduchon metrics of compact complex manifolds, J. Geom. Anal.25(2015), no. 3, 2011–2027. ↑3, 12
work page 2015
-
[60]
T. Yamada,A construction of compact pseudo-K¨ ahler solvmanifolds with no K¨ ahler structures, Tsukuba J. Math.29 (2005), no. 1, 79–109.↑4 (Ettore Lo Giudice)Dipartimento di Scienze Matematiche, Fisiche e Informatiche Unit`a di Matematica e Infor- matica, Universit`a degli Studi di Parma, Parco Area delle Scienze 53/A, 43124, Parma, Italy &, Dipartimento ...
work page 2005
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.