u(x, 0) I/J(X) at t = 0.
The function ip(x) is assumed to be everywhere continuous and
bounded. It being assumed that both u and x/j as functions of x
are periodic of period 1.
From physical considerations based on the superposition principle and the idea of
separation of variables one is lead to the two solutions
u(x,t)= / VK0{l + 2 y -
4 M
c o s 2 ( x - £ ) H
* „=i
1 oo
*o{1^ £ .-~-"*'«H

Using an elementary lemma from the calculus of variations, the uniqueness of the
solution u, itself a consequence of the energy estimate ^ JQ
0, leads imme-
diately to an equivalent form of the theta formula. The functional equation of the
Riemann zeta function in its usual form is then obtained by applying a suitable
Mellin transform to both sides of the theta identity.
New ideas in functional analysis and the rise of the theory of distributions made
it possible to develop the above elementary argument into a powerful technique
capable of new applications to two closely related branches of mathematics: to zeta
and L-functions and to infinite dimensional group representations.
Weil was the first to formulate in the local to global language of Tate's Thesis
the relation between functional equations and the uniqueness of certain zeta distri-
butions. At the heart of his new interpretation is an old idea of Weil concerning
the determination of relative invariant measures, which had already appeared in his
classic work on integration over topological groups. The subsequent full develop-
ment of these ideas by Weil himself, and the light they shed on Siegel's insights into
the Poisson Summation Formula, established beyond doubt the fruitfulness of this
new point of view. Chapter I is an elementary introduction to this circle of ideas.
It includes a detailed presentation of Tate's Thesis together with Weil's ideas about
distributions and zeta functions. In this framework, the local calculations at the
infinite prime are simply an elaboration of the well known results of Hadamard and
M. Riesz on homogeneous distributions. Our elementary presentation can serve as
an introduction to the theories of Sato concerning hyperfunctions defined by com-
plex powers of polynomials and zeta functions on prehomogeneous spaces and to
the theory of the Bernstein polynomial. We have also given a brief outline of the
Jacquet-Godement theory of principal L-functions which is the natural generaliza-
tion to GL(n) of Hecke's L-functions viewed as automorphic objects on GL(1). The
principal results of Chapter I are those of Tate and Weil concerning the uniqueness
of local and global zeta distributions (Lemma 9, Theorem 22) and the functional
equation (Theorem 23).
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