mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...

, Eichler cohomology (also called parabolic cohomology or cuspidal cohomology) is a cohomology theory for Fuchsian groups, introduced by , that is a variation of group cohomology analogous to the image of the

cohomology with compact support In mathematics, cohomology with compact support refers to certain cohomology theories, usually with some condition requiring that cocycles should have compact support. Singular cohomology with compact support Let X be a topological space. Then :\d ...

in the ordinary cohomology group. The Eichler–Shimura isomorphism, introduced by Eichler for complex cohomology and by for real cohomology, is an isomorphism between an Eichler cohomology group and a space of cusp forms. There are several variations of the Eichler–Shimura isomorphism, because one can use either real or complex coefficients, and can also use either Eichler cohomology or ordinary group cohomology as in . There is also a variation of the Eichler–Shimura isomorphisms using ''l''-adic cohomology instead of real cohomology, which relates the coefficients of cusp forms to eigenvalues of Frobenius acting on these groups. used this to reduce the Ramanujan conjecture to the Weil conjectures that he later proved.

Eichler cohomology

If ''G'' is a Fuchsian group and ''M'' is a representation of it then the Eichler cohomology group ''H''(''G'',''M'') is defined to be the kernel of the map from ''H''(''G'',''M'') to Π_''c'' ''H''(''G''_''c'',''M''), where the product is over the cusps ''c'' of a fundamental domain of ''G'', and ''G''_''c'' is the subgroup fixing the cusp ''c''.

References

* * * * * {{DEFAULTSORT:Eichler-Shimura isomorphism Modular forms