Mitchell J. Feigenbaum, a pioneer in the field of mathematical physics known as chaos, died on June 30 in Manhattan. He was 74. ... During a postdoctoral fellowship at Cornell, Dr. Feigenbaum was already allowing his focus to wander. “He was not very happy with the physics he was doing at that time,” said Predrag Cvitanovic, a theoretical physicist at the Georgia Institute of Technology School of Physics, who was a graduate student at Cornell at the time. “The main thing he did is he solved the New York Times crossword puzzle every morning,” Dr. Cvitanovic said. “It was just clear that he was very smart. He didn’t produce anything I was aware of.”