  This is a rework of our old file, which has been left unpublished since
September 1994, on an explicit spectral decomposition of the fourth power
moment of the Riemann zeta-function against a weight which is the square of a
Dirichlet polynomial. At this occasion we add an explicit treatment of
generalized Kloosterman sums associated with arbitrary Hecke congruence
subgroups (Section 15), which might have an independent interest. At the end
(Section 36) of our discussion, we set out a few problems on the distribution
of eigenvalues of the hyperbolic Laplacian, which appear to us to be related to
the nature of the sixth power moment of the Riemann zeta-function. The contents
of this work were presented in a worshop at RIMS Kyoto University on October
18, 2007.
  In this second version, some corrections are made in the part on generalized
Kloosterman sums, and in Addendum a mention is made concerning a recent work by
C.P. Hughes and M.P. Young (0709.2345).
