Description: Papers of Mahlon Marsh Day (1913-92), professor of Mathematics (1940-83), including publications, correspondence, manuscripts, reviews, grant proposals, research notes, course materials, and reprints, concerning Day's graduate work at Brown University (1934-40) and the University of Chicago, scholarly conferences (1950-84), departmental committees (1959-83), Day's editorship of the Illinois Journal of Mathematics (1968-73, 1981-85), and talks given at other colleges (1960-86). Day's research interests included functional analysis, linear spaces, ordered systems, geometry of normal spaces, and amenable semigroups. Significant correspondence include Alan L.T. Paterson, Juan J. Schaffer, and Elizabeth Coone Day. The series includes a group photograph of the 42nd summer meeting of the Mathematical Organization of America (1936).