József Hajnal was a Hungarian mathematician known for his significant contributions to set theory and combinatorics. He is best recognized for the Hajnal-Szemerédi theorem, which addresses the structure of large graphs and has implications in various areas of mathematics. Born in 1924, Hajnal's work has influenced many fields, including graph theory and mathematical logic. He collaborated with other prominent mathematicians and published numerous papers throughout his career, helping to advance the understanding of complex mathematical concepts. His legacy continues to inspire new generations of mathematicians.