This is a rework of our old file on an explicit spectral decomposition of the mean value
$$M_2(g;A)=\int_{-\infty}^\infty{\left|{\zeta({\textstyle{1\over2}}+it)}\right|^{4}\left|{A({\textstyle{1\over2}}+it)}\right|^{2}g(t)dt}$$
that has been left unpublished since September 1994, though its summary account is given in [9] (see also [11, Section 4.6]); here
$$A(s)=\sum\limits_n{\alpha_{n}n^{-s}}$$
is a finite Dirichlet series and g is assumed to be even, regular, real-valued on R, and of fast decay on a sufficiently wide horizontal strip. On this occasion we add greater details as well as a rigorous treatment of the Mellin transform
$$Z_2(s;A)=\int_1^\infty{\left|{\zeta({\textstyle{1\over2}}+it)}\right|^{4}\left|{A({\textstyle{1\over2}}+it)}\right|^{2}t^{-s}dt}$$
which was scantly touched on in [9]. In particular, we specify the location of its poles and respective residues, under a mild condition on the coefficients $\alpha_n$.
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