On the zeros of linear combinations of L-functions of degree two on the critical line. Selberg's approach

We consider in detail Selberg's method for proving that under certain natural assumptions, a positive proportion of the non-trivial zeros of a linear combination of L-functions from the Selberg class lie on the critical line. As an example, we provide all the ingredients necessary to prove this result in the case of a linear combination of L-functions of degree two attached to automorphic forms.