Can anyone help my long term memory on the following? When I was a research student at Oxford 1956-59 people were saying that category theory was a soft subject, and mathematicians should tackle hard problems, an example of which given was a problem in combinatorics. My memory is that it then turned out that this was solved using the notion of free category on a graph which allowed for an inductive procedure. The paper was I think published in a prestigious journal (Proc. Nat. Acad, Sci.?). Is my memory playing tricks or can someone make this precise? Ronnie Brown -- Professor Emeritus R. Brown, http://www.bangor.ac.uk/~mas010/ (Links to survey articles: Higher dimensional group theory Groupoids and crossed objects in algebraic topology) Raising Public Awareness of Mathematics CDRom Version 1.1 http://www.bangor.ac.uk/~mas010/CDadvert.html Symbolic Sculpture and Mathematics: http://www.cpm.informatics.bangor.ac.uk/sculmath/ Centre for the Popularisation of Mathematics http://www.cpm.informatics.bangor.ac.uk/ 13-May-2002 07:32:31 -0300,1524;000000000001-00000000