= András Hajnal
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<András Hajnal> was a <mathematician> working in <set theory> and <combinatorics>. He served as director of DIMACS and later as a Rutgers mathematics professor before returning to Hungary after retirement.https://www.math.rutgers.edu/people/department-directory/detail/343-in-memoriam/2066-hajnal-andras[Institutional memorial, 1931–2016 and career]{ref}
One result bearing his name, proved with <Paul Erdős>, says that every infinite <set> admits an <omega-Jónsson function>: a colouring of its countably infinite <subsets> by its elements such that every subset of the same <cardinality> still sees every colour. Assuming the <axiom of choice>, even restricting to a large subset cannot simplify this colouring by omitting a colour. This illustrates the negative side of infinite <partition relations>, complementing <Ramsey theory> results that force homogeneous subsets.<Partition calculus (3) by Andrés Caicedo>[Definition 1 and Theorem 2, Erdős-Hajnal theorem]{ref}
\Image[https://www.math.rutgers.edu/images/people/hajnal.jpeg]
{title=András Hajnal, Rutgers memorial portrait}
{height=240}
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