András Hajnal was a prominent Hungarian mathematician known for his contributions to set theory and combinatorics. Born on May 18, 1925, he made significant advancements in the field, particularly in the study of infinite graphs and the theory of partially ordered sets. His work has influenced various areas of mathematics and has been widely recognized. Hajnal is perhaps best known for the Hajnal-Szemerédi theorem, which addresses the properties of large subsets of graphs. Throughout his career, he published numerous papers and collaborated with other mathematicians, leaving a lasting impact on the mathematical community before his passing on January 12, 2021.