The word “solutions” is much overused, even misused, now. When I see a van with “Cleaning solutions” on the side, I imagine it full of containers of ammonia or soapy water, while “Printing solutions” can only mean ink … But here I am talking about “solutions to the exercises”.

*Designs, Graphs, Codes and their Links* is the only book I wrote jointly. It came about when Jack van Lint and I were separately invited by Dan Hughes to lecture at Westfield College on things connected to designs (the audience, including Fred Piper and Marion Kimberley, were experts on design theory). I chose graphs, since this was soon after I had written a thesis on the Higman–Sims graph and related things. Jack knew about codes, which give rise to designs by taking the supports of codewords of given weight.

After the lectures, Dan suggested that we should publish them jointly as a book in the London Mathematical Society Lecture Note series. We put them together with minimal editing; I think the typing was done at Westfield.

The book went through several versions. The publishers, Cambridge University Press, preferred a new title to a new edition with the same title, so we went through several permutations of graphs, codes and designs. Finally in 1990 the Press proposed moving the book to the Student Texts series. This meant, as well as a major revision, adding a large number of exercises. We planned this at the Marshall Hall memorial conference in Burlington, Vermont, in 1990, singlemindedly missing the excursion (which included a trip to Ben and Jerry’s factory) to sit in a motel room all afternoon dividing up the workload and suggesting new topics and problems.

The book contains material which can’t easily be found elsewhere. But if there were to be a new edition, it would need another major rewrite. For example, we treat Kerdock codes and the associated binary geometry; but we were too early for the major paper of Hammons *et al.* on quaternary codes and the Gray map, followed by Calderbank *et al.* on the connections with real and complex line systems, from which there is a direct link to mutually unbiased bases in quantum computing.

At some point, somebody emailed me to ask whether there were solutions to the exercises. I sat down and wrote out solutions to the exercises in the first eight chapters (essentially, the ones I wrote), and at some later point I typed these up in LaTeX.

I had completely forgotten about this until, going through old files, I came upon them. I could make them public now, though they will need a careful check first.

I’m not rushing in because I know there are different views on solutions to exercises. Basically, if you are studying the book on your own, they are very helpful, though there is always a temptation to look at the solutions too soon, or to say “I know how to do that, I will just check the solution” rather than engaging fully with the problem. On the other hand, many instructors using a textbook like to assign problems to the class, and the last thing they want is to have the solutions available to the students.

I suspect that DGCL is used more in the first way than in the second. Any thoughts?

Solutions are welcome and I believe, they are necessary. Not everybody is privileged to take a course and have an instructor. For somebody who is studying independently, solution is an indicator of their progress and understanding.

About the tendency to see the solutions before trying: All one can do take the horse to the pond. Somebody who wants to study seriously knows when to see the solution.

About the fact that some students could cheat by copying: Copying/cheating is definitely wrong. But its the instructor’s choice (or say laziness), to whether mark some problems from a textbook as an assignment or give new questions (sometimes just be twisting some simple things in the problem).

The bigger problem is ‘textbook mafia’, it fallouts like unnecessary new editions, restricted ‘instructor’s manual’ etc. My argument is not for an open source book or a free book. Please charge for, it but make things legally available. I hope the professors (who were one time students) understand that a student after a reasonable effort, need not break his head unnecessarily over a problem when he/she can look up the solution and then, proceed to further learning.