Almgren isomorphism theorem

Almgren isomorphism theorem is a result in geometric measure theory and algebraic topology about the topology of the space of flat cycles in a Riemannian manifold.

The theorem plays a fundamental role in the Almgren–Pitts min-max theory as it establishes existence of topologically non-trivial families of cycles, which were used by Frederick J. Almgren Jr., Jon T. Pitts and others to prove existence of (possibly singular) minimal submanifolds in every Riemannian manifold. In the special case of codimension 1 cycles with mod 2 coefficients Almgren isomorphism theorem implies that the space of cycles is weakly homotopy equivalent to the infinite real projective space.[1]

## Statement of the theorem

Let M be a Riemannian manifold. Almgren isomorphism theorem asserts that the m-th homotopy group of the space of flat k-dimensional cycles in M is isomorphic to the (m+k)-th dimensional homology group of M. This result is a generalization of the Dold–Thom theorem, which can be thought of as the k=0 case of Almgren's theorem.[2]

The isomorphism is defined as follows. Let G be an abelian group and Zk(M;G) denote the space of flat cycles with coefficients in group G. To each family of cycles f:Sm->Zk(M;G) we associate an (m+k)-cycle C as follows. Fix a fine triangulation T of Sm. To each vertex v in the 0-skeletion of T we associate a cycle f(v). To each edge E in the 1-skeleton of T with ∂E=v-w we associate a (k+1)-chain with boundary f(v)-f(w) of minimal mass. We proceed this way by induction over the skeleton of T. The sum of all chains corresponding to m-dimensional faces of T will be the desired (m+k)-cycle C. Even though the choices of triangulation and minimal mass fillings were not unique, they all result in an (m+k)-cycle in the same homology class.[3]

## References

1. ↑ *White, Brian (1998), "The mathematics of F. J. Almgren, Jr.", The Journal of Geometric Analysis 8 (5): 681–702, doi:10.1007/BF02922665, ISSN 1050-6926
2. ↑ F. J. Almgren. The Homotopy Groups of the Integral Cycle Groups. PhD thesis, 1962
3. ↑ Guth, L. The Width-Volume Inequality. GAFA Geom. funct. anal. 17, 1139–1179 (2007)

## Further reading

* F. J. Almgren. The Homotopy Groups of the Integral Cycle Groups. PhD thesis, 1962. OCLC: 22016723
* A. Neves, "New applications of Min-max Theory", Proceedings of International Congress of Mathematics, (2014), 939-957
* F. C. Marques, A. Neves. "Applications of Almgren--Pitts Min-max Theory", Current developments in mathematics, 1-71, Int. Press, Somerville, MA, 2014
* White, Brian (1997), "The Mathematics of F. J. Almgren Jr.", Notices of the American Mathematical Society 44 (11): 1451–1456, ISSN 0002-9920, https://www.ams.org/notices/199711/index.html

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