He was one of the pioneers in the field of computational complexity theory, and he also worked in the field of distributed computing.
en.wikipedia.org Theoretical computer science includes computability theory, computational complexity theory, and information theory.
en.wikipedia.org Here, easy and hard are to be understood in the sense of computational complexity theory, specifically the theory of polynomial time problems.
en.wikipedia.org A study of computational complexity theory helps to motivate combinatorial search.
en.wikipedia.org His areas of research include combinatorics, randomness, pseudorandomness, information theory, communication complexity, computational complexity theory, quantum computation, quantum information science.
en.wikipedia.org