Well, for me it is strange, but I still greatly respect and admire their work. Perhaps this has to do with different cultural standards. In Holland they often call one’s Graduate final work as ‘a book’ more than a ‘PhD thesis’. There are some subtle differences between these two terms. I was raised in a tradition under which a PhD theis is not the same as a book, and which requires a PhD thesis to have one single author.

In any case, of course, anything that has been written by Groenendijk and/or Stokhof can be considered hors concours.

I think that first of all it is a matter of rules respecting the kind of work that is in question. For instance, under the criteria of intellectual honesty and academic propiety, co-authored papers or books are ok, but a co-authored PhD thesis is simply out of question.

In the are of Mathematics some Institutions in Brazil have adopted a point based system to evaluate Faculty members’ production: a single author paper counts as 1 paper in his production, a two author paper counts as 1/2 paper in each author’s production, a three author paper as 1/3 and so on.