Proof verification and hardness of approximation problems. M. Bellare, S. Goldwasser, C. Lund, and A. Russell. Efficient probabilistically checkable proofs and applications to approximation. Michael Ben-Or, Shafi Goldwasser, Joe Kilian, and Avi Wigderson. Uriel Feige and Joe Kilian.
Impossibility results for recycling random bits in two-prover proof systems.
David S. Johnson. Approximation algorithms for combinatorial problems. Carsten Lund and Mihalis Yannakakis. On the hardness of approximating minimization problems. Moni Naor, Leonard J. Schulman, and Aravind Srinivasan.