One by one, alas, we are slowly being whittled away. I mourn this loss. -- Fred Linton ________________________________________ From: RONALD BROWN [ronnie.profbrown@btinternet.com] Sent: Wednesday, April 22, 2015 05:59 To: categories@mta.ca Subject: categories: Philip J Higgins I heard yesterday, April 21, that Philip Higgins died on 25 March, peacefully in an armchair, from heart conditions. His 1964 paper on "Presentations of groupoids with applications to groups" introduced me to the idea that groupoids could be used for algebraic computations. A key construction in this paper is that of a new groupoid U_f(G) over Y from a groupoid G and a function f: Ob(G) --> Y. This construction easily gives free groups and free products of groups, and has important higher dimensional generalisations. This paper led me to a Seifert-van Kampen theorem for the fundamental grouoid on a set of base points, allowing applications to unions of spaces for which they or their intersections are not connected, such as the circle. This work led in 1974 to a long term collaboration on higher groupoids and their applications. He had a wonderful way of organising complex algebraic material, and much of this expository skill is shown in the book R. Brown, P.J. Higgins, R. Sivera, "Nonabelian algebraic topology: filtered spaces, crossed complexes, cubical homotopy groupoids", EMS Tracts in Mathematics Vol. 15, 703 pages. (August 2011). where major central parts are lifted from our joint papers. When he got into a new subject, he saw ways of clarifying the matter and exposition. This is shown in his work with Kirill Mackenzie on Lie groupoids and algebroids. His early work on groups with operators, and on the algebraic theory of algebras with partial operations, are still refereed to. At one time he had to choose between two professions, and decided he could have more fun as a professional mathematician and an amateur violinist. His craftmanship is shown that in his retirement he took up again making for his grandchildren violins, violas and cellos, each one taking about a year. I miss a good friend. Ronnie Brown [For admin and other information see: http://www.mta.ca/~cat-dist/ ]