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 1 Problem
If a compact Lie group acts smoothly and non-trivially on a closed smooth manifold , what constraints does this place on the topology of in general and on the Pontrjagin classes of in particular? In the case where is homotopy equivalent to , , Petrie [Petrie1972] restricted his attention to actions of the Lie group and proved that if the fixed point set of the action consists only of isolated fixed points, then the Pontrjagin classes of are determined by the representations of at the fixed points. Motivated by this result, Petrie [Petrie1972] posed the following conjecture.
Conjecture 1.1 (Petrie conjecture). Suppose that acts smoothly and non-trivially on a closed smooth -manifold . Then the total Pontrjagin class of agrees with that of , i.e., for a generator of .
 2 Progress to date
As of December 21, 2010, the Petrie conjecture has not been confirmed in general. However, it has been proven in the following special cases.
- Petrie [Petrie1973] has verified his conjecture under the assumption that the manifold admits a smooth action of the torus .
- By the work of [Dejter1976], the Petrie conjecture is true if , i.e., and hence, if .
- Related results go back to [Musin1978] and [Musin1980], in particular, the latter work shows that the Petrie conjecture holds if , i.e., .
- According to [Hattori1978], the Petrie conjecture holds if admits an invariant almost complex structure with the first Chern class .
- Other special cases where the Petrie conjecture holds are described by [Wang1975], [Yoshida1975/76], [Iberkleid1978].
- By [Tsukada&Washiyama1979] and [Masuda1981], the Petrie conjecture is true if the fixed point set consists of three or four connected components.
- Masuda [Masuda1983] proved the Petrie conjecture in the case where admits a specific smooth action of for .
- The work of [James1985] confirms the result of [Musin1980] that the Petrie conjecture is true if , i.e., .
- According to [Dessai2002], the Petrie conjecture holds if admits an appropriate smooth action of and .
- It follows from [Dessai&Wilking2004] that the Petrie conjecture holds if admits a smooth action of and .
 3 Further discussion
Masuda and Suh [Masuda&Suh2008] posed the following question about the invariance of Pontrjagin classes for toric -manifolds.
Question 3.1. For two toric -manifolds with isomorphic cohomology rings, is it true that any isomorphism between the cohomology rings preserves the Pontrjagin classes of the two manifolds?
A symplectic version of the Petrie conjecture is discussed by Tolman [Tolman2010]. In particular, the following question has been posed.
Question 3.2. If the circle acts in a Hamiltonian way on a compact symplectic manifold with for all , is it true that for all ? Is the total Chern class of determined by the cohomology ring ?
 4 References
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- [Dessai&Wilking2004] A. Dessai and B. Wilking, Torus actions on homotopy complex projective spaces, Math. Z. 247 (2004), no.3, 505–511. MR2114425 (2006c:57033) Zbl 1068.57034
- [Dessai2002] A. Dessai, Homotopy complex projective spaces with -action, Topology Appl. 122 (2002), no.3, 487–499. MR1911696 (2003f:58048) Zbl 0998.57048
- [Hattori1978] A. Hattori, -structures and -actions, Invent. Math. 48 (1978), no.1, 7–31. MR508087 (80e:57051) Zbl 0395.57020
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- [Tolman2010] S. Tolman, On a symplectic generalization of Petrie's conjecture, Trans. Amer. Math. Soc. 362 (2010), no.8, 3963–3996. MR2638879 Zbl 1216.53074
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- [Wang1975] K. Wang, Differentiable circle group actions on homotopy complex projective spaces, Math. Ann. 214 (1975), 73–80. MR0372895 (51 #9099) Zbl 0285.57025
- [Yoshida1975/76] T. Yoshida, On smooth semifree actions on cohomology complex projective spaces, Publ. Res. Inst. Math. Sci. 11 (1975/76), no.2, 483–496. MR0445528 (56 #3868) Zbl 0326.57008