C. S. Rajan's most significant contribution to number theory was his work on modular forms. He developed a new method for constructing modular forms, which led to a number of important results in the field.

A modular form is a function that is invariant under the action of a group of matrices. This means that if you apply any matrix in the group to the function, the function will remain unchanged.

An automorphic form is a function that is invariant under the action of a group of matrices. This means that if you apply any matrix in the group to the function, the function will remain unchanged.

C. S. Rajan's work on modular forms was used to construct new modular forms. This led to a number of important results in the field, including the discovery of new types of modular forms and the development of new methods for studying modular forms.

C. S. Rajan's work on automorphic forms was used to construct new automorphic forms. This led to a number of important results in the field, including the discovery of new types of automorphic forms and the development of new methods for studying automorphic forms.

C. S. Rajan's most famous result was the Ramanujan-Petersson conjecture. This conjecture was first proposed by Srinivasa Ramanujan in 1916, and it was eventually proved by C. S. Rajan in 1973.

The Ramanujan-Petersson conjecture is a conjecture about the existence of new types of modular forms. It was first proposed by Srinivasa Ramanujan in 1916, and it was eventually proved by C. S. Rajan in 1973.

C. S. Rajan was a major contributor to the development of the Langlands program. He made a number of important contributions to the program, including the development of new methods for studying automorphic forms and the discovery of new types of modular forms.

The Langlands program is a program to unify number theory and representation theory. It was first proposed by Robert Langlands in 1967, and it has since become one of the most important and influential programs in mathematics.

C. S. Rajan was a major contributor to the development of the modularity theorem. He made a number of important contributions to the theorem, including the development of new methods for studying modular forms and the discovery of new types of modular forms.

The modularity theorem is a theorem that relates modular forms to elliptic curves. It was first proposed by Andrew Wiles in 1993, and it was eventually proved by Wiles and Richard Taylor in 1995.

C. S. Rajan was a major contributor to the development of the trace formula for automorphic forms. He made a number of important contributions to the formula, including the development of new methods for studying automorphic forms and the discovery of new types of modular forms.

The trace formula for automorphic forms is a formula that relates the trace of an automorphic form to its Fourier coefficients. It was first proposed by Atle Selberg in 1956, and it has since become one of the most important and influential formulas in mathematics.

C. S. Rajan was a major contributor to the development of the theory of automorphic forms. He made a number of important contributions to the theory, including the development of new methods for studying automorphic forms and the discovery of new types of modular forms.

The theory of automorphic forms is a theory that studies the properties of automorphic forms. It is a vast and complex theory, and it has applications in a wide variety of areas of mathematics, including number theory, representation theory, and algebraic geometry.