In mathematics, **topological modular forms (tmf)** is the name of a spectrum that describes a generalized cohomology theory. In concrete terms, for any integer *n* there is a topological space , and these spaces are equipped with certain maps between them, so that for any topological space *X*, one obtains an abelian group structure on the set of homotopy classes of continuous maps from *X* to . One feature that distinguishes tmf is the fact that its coefficient ring, (point), is almost the same as the graded ring of holomorphic modular forms with integral cusp expansions. Indeed, these two rings become isomorphic after inverting the primes 2 and 3, but this inversion erases a lot of torsion information in the coefficient ring.

The spectrum of topological modular forms is constructed as the global sections of a sheaf of E-infinity ring spectra on the moduli stack of (generalized) elliptic curves. This theory has relations to the theory of modular forms in number theory, the homotopy groups of spheres, and conjectural index theories on loop spaces of manifolds. tmf was first constructed by Michael Hopkins and Haynes Miller; many of the computations can be found in preprints and articles by Paul Goerss, Hopkins, Mark Mahowald, Miller, Charles Rezk, and Tilman Bauer.

The original construction of tmf uses the obstruction theory of Hopkins, Miller, and Paul Goerss, and is based on ideas of Dwyer, Kan, and Stover. In this approach, one defines a presheaf O^{top} ("top" stands for topological) of multiplicative cohomology theories on the etale site of the moduli stack of elliptic curves and shows that this can be lifted in an essentially unique way to a sheaf of E-infinity ring spectra. This sheaf has the following property: to any etale elliptic curve over a ring R, it assigns an E-infinity ring spectrum (a classical elliptic cohomology theory) whose associated formal group is the formal group of that elliptic curve.

A second construction, due to Jacob Lurie, constructs tmf rather by describing the moduli problem it represents and applying general representability theory to then show existence: just as the moduli stack of elliptic curves represents the functor that assigns to a ring the category of elliptic curves over it, the stack together with the sheaf of E-infinity ring spectra represents the functor that assigns to an E-infinity ring its category of oriented derived elliptic curves, appropriately interpreted. These constructions work over the moduli stack of smooth elliptic curves, and they also work for the Deligne-Mumford compactification of this moduli stack, in which elliptic curves with nodal singularities are included. TMF is the spectrum that results from the global sections over the moduli stack of smooth curves, and tmf is the spectrum arising as the global sections of the Deligne–Mumford compactification.

TMF is a periodic version of the connective tmf. While the ring spectra used to construct TMF are periodic with period 2, TMF itself has period 576. The periodicity is related to the modular discriminant.

Some interest in tmf comes from string theory and conformal field theory. Graeme Segal first proposed in the 1980s to provide a geometric construction of elliptic cohomology (the precursor to tmf) as some kind of moduli space of conformal field theories, and these ideas have been continued and expanded by Stephan Stolz and Peter Teichner. Their program is to try to construct TMF as a moduli space of supersymmetric Euclidean field theories.

In work more directly motivated by string theory, Edward Witten introduced the Witten genus, a homomorphism from the string bordism ring to the ring of modular forms, using equivariant index theory on a formal neighborhood of the trivial locus in the loop space of a manifold. This associates to any spin manifold with vanishing half first Pontryagin class a modular form. By work of Hopkins, Matthew Ando, Charles Rezk and Neil Strickland, the Witten genus can be lifted to topology. That is, there is a map from the string bordism spectrum to tmf (a so-called *orientation*) such that the Witten genus is recovered as the composition of the induced map on the homotopy groups of these spectra and a map of the homotopy groups of tmf to modular forms. This allowed to prove certain divisibility statements about the Witten genus. The orientation of tmf is in analogy with the Atiyah–Bott–Shapiro map from the spin bordism spectrum to classical K-theory, which is a lift of the Dirac equation to topology.

In mathematics, specifically in homology theory and algebraic topology, **cohomology** is a general term for a sequence of abelian groups associated to a topological space, often defined from a cochain complex. Cohomology can be viewed as a method of assigning richer algebraic invariants to a space than homology. Some versions of cohomology arise by dualizing the construction of homology. In other words, cochains are functions on the group of chains in homology theory.

In mathematics, a **scheme** is a mathematical structure that enlarges the notion of algebraic variety in several ways, such as taking account of multiplicities and allowing "varieties" defined over any commutative ring.

In algebraic topology, a branch of mathematics, a **spectrum** is an object representing a generalized cohomology theory. There are several different categories of spectra, but they all determine the same homotopy category, known as the **stable homotopy category**.

In mathematics, the **Thom space,****Thom complex,** or **Pontryagin–Thom construction** of algebraic topology and differential topology is a topological space associated to a vector bundle, over any paracompact space.

In mathematics, **elliptic cohomology** is a cohomology theory in the sense of algebraic topology. It is related to elliptic curves and modular forms.

**Jack Johnson Morava** is an American homotopy theorist at Johns Hopkins University.

In mathematics a **stack** or **2-sheaf** is, roughly speaking, a sheaf that takes values in categories rather than sets. Stacks are used to formalise some of the main constructions of descent theory, and to construct fine moduli stacks when fine moduli spaces do not exist.

**Michael Jerome Hopkins** is an American mathematician known for work in algebraic topology.

In mathematics, a **highly structured ring spectrum** or -ring is an object in homotopy theory encoding a refinement of a multiplicative structure on a cohomology theory. A commutative version of an -ring is called an -ring. While originally motivated by questions of geometric topology and bundle theory, they are today most often used in stable homotopy theory.

**Mark Edward Mahowald** was an American mathematician known for work in algebraic topology.

In mathematics, the **Landweber exact functor theorem**, named after Peter Landweber, is a theorem in algebraic topology. It is known that a complex orientation of a homology theory leads to a formal group law. The Landweber exact functor theorem can be seen as a method to reverse this process: it constructs a homology theory out of a formal group law.

This is a **glossary of algebraic geometry**.

In algebraic geometry, a **quotient stack** is a stack that parametrizes equivariant objects. Geometrically, it generalizes a quotient of a scheme or a variety by a group: a quotient variety, say, would be a coarse approximation of a quotient stack.

In the mathematical field of algebraic topology, a **commutative ring spectrum**, roughly equivalent to a -ring spectrum, is a commutative monoid in a good category of spectra.

**Derived algebraic geometry** is a branch of mathematics that generalizes algebraic geometry to a situation where commutative rings, which provide local charts, are replaced by either differential graded algebras, simplicial commutative rings or -ring spectra from algebraic topology, whose higher homotopy groups account for the non-discreteness of the structure sheaf. Grothendieck's scheme theory allows the structure sheaf to carry nilpotent elements. Derived algebraic geometry can be thought of as an extension of this idea, and provides natural settings for intersection theory of singular algebraic varieties and cotangent complexes in deformation theory, among the other applications.

In algebraic geometry, given a smooth projective curve *X* over a finite field and a smooth affine group scheme *G* over it, the **moduli stack of principal bundles** over *X*, denoted by , is an algebraic stack given by: for any -algebra *R*,

In topology, a discipline within mathematics, the **Brown–Gitler spectrum** is a spectrum whose cohomology is a certain cyclic module over the Steenrod algebra.

**Charles Waldo Rezk** is an American mathematician, specializing in algebraic topology, category theory, and spectral algebraic geometry.

- Bauer, T., Computation of the homotopy of the spectrum tmf (2008), https://arxiv.org/abs/math.AT/0311328
- Behrens, M., Notes on the Construction of tmf (2007), http://www-math.mit.edu/~mbehrens/papers/buildTMF.pdf
- Douglas, Christopher L.; Francis, John; Henriques, André G.; et al., eds. (2014),
*Topological Modular Forms*, Mathematical Surveys and Monographs,**201**, A.M.S., ISBN 978-1-4704-1884-7

- Goerss, P. and Hopkins, M., Moduli Spaces of Commutative Ring Spectra, http://www.math.northwestern.edu/~pgoerss/papers/sum.pdf
- Hopkins, M., Algebraic Topology and Modular Forms (2002), https://arxiv.org/abs/math.AT/0212397
- Hopkins, M and Mahowald, M., From Elliptic Curves to Homotopy Theory (1998), http://www.math.purdue.edu/research/atopology/Hopkins-Mahowald/eo2homotopy.pdf
- Lurie, J, A Survey of Elliptic Cohomology (2007), http://www.math.harvard.edu/~lurie/papers/survey.pdf
- Rezk, C., http://www.math.uiuc.edu/~rezk/512-spr2001-notes.pdf
- Stolz, S. and Teichner, P., Supersymmetric Euclidean Field theories and generalized cohomology (2008), http://math.berkeley.edu/~teichner/Papers/Survey.pdf

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