X-Nico

Robert Langlands

The book by Hervé Jacquet and Langlands on $GL(2)$ presented a theory of automorphic forms for the general linear group $GL(2)$, establishing among other things the Jacquet–Langlands correspondence showing that functoriality was capable of explaining very precisely how automorphic forms for $GL(2)$ related to those for quaternion algebras.

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This book applied the adelic trace formula for $GL(2)$ and quaternion algebras to do this.

Steven Gaal

Robert Langlands has cited Gaal's influence in his early investigations of zeta functions and Eisenstein series.

Unifying theories in mathematics

In fact the Langlands program (or philosophy) is much more like a web of unifying conjectures; it really does postulate that the general theory of automorphic forms is regulated by the L-groups introduced by Robert Langlands.

see also