Eisenstein integral

In mathematical representation theory, the Eisenstein integral is an integral introduced by Harish-Chandra (1970, 1972) in the representation theory of semisimple Lie groups, analogous to Eisenstein series in the theory of automorphic forms. Harish-Chandra (1975, 1976a, 1976b) used Eisenstein integrals to decompose the regular representation of a semisimple Lie group into representations induced from parabolic subgroups. Trombi (1989) gave a survey of Harish-Chandra's work on this.

Definition

Harish-Chandra (1970, section 10) defined the Eisenstein integral by

$\displaystyle E(P:\psi:\nu:x) = \int_K\psi(xk)\tau(k^{-1})\exp((i\nu-\rho_P)H_P(xk)) \, dk$

where:

x is an element of a semisimple group G
P = MAN is a cuspidal parabolic subgroup of G
ν is an element of the complexification of a
a is the Lie algebra of A in the Langlands decomposition P = MAN.
K is a maximal compact subgroup of G, with G = KP.
ψ is a cuspidal function on M, satisfying some extra conditions
τ is a finite dimensional unitary double representation of K
H_P(x) = log a where x = kman is the decomposition of x in G = KMAN.

References

Harish-Chandra (1970), "Harmonic analysis on semisimple Lie groups", Bulletin of the American Mathematical Society 76: 529–551, doi:10.1090/S0002-9904-1970-12442-9, ISSN 0002-9904, MR 0257282
Harish-Chandra (1972), "On the theory of the Eisenstein integral", in Gulick, Denny; Lipsman, Ronald L., Conference on Harmonic Analysis (Univ. Maryland, College Park, Md., 1971), Lecture Notes in Mathematics, 266, Berlin, New York: Springer-Verlag, pp. 123–149, doi:10.1007/BFb0059640, ISBN 978-3-540-05856-4, MR 0399355
Harish-Chandra (1975), "Harmonic analysis on real reductive groups. I. The theory of the constant term", J. Functional Analysis 19: 104–204, doi:10.1016/0022-1236(75)90034-8, MR 0399356
Harish-Chandra (1976a), "Harmonic analysis on real reductive groups. II. Wavepackets in the Schwartz space", Inventiones Mathematicae 36: 1–55, doi:10.1007/BF01390004, ISSN 0020-9910, MR 0439993
Harish-Chandra (1976b), "Harmonic analysis on real reductive groups. III. The Maass-Selberg relations and the Plancherel formula", Annals of Mathematics. Second Series 104 (1): 117–201, ISSN 0003-486X, MR 0439994, http://www.jstor.org/stable/1971058
Trombi, P. C. (1989), "On Harish-Chandra's theory of the Eisenstein integral for real semisimple Lie groups", in Sally, Paul J.; Vogan, David A., Representation theory and harmonic analysis on semisimple Lie groups, Math. Surveys Monogr., 31, Providence, R.I.: American Mathematical Society, pp. 287–350, ISBN 978-0-8218-1526-7, MR 1011900, http://books.google.com.do/books?id=5oJG0ukOVswC