The MacWilliams identities relate the weight enumerators of a code and its dual code. I treated the version for linear codes in my combinatorics class, but felt unsatisfied because I didn't have a use for them (and judging from the feedback, some students shared this feeling).


So... do you know of any neat application of the MacWilliams relations? Preferably one that I can treat in one lecture. I'm thinking about trying to prove the non-existence of codes with certain parameters, but anything is welcome.


One application that I'm aware of is the role these relations played in the proof of the nonexistence of a projective plane of order 10, but this is not ideal material for a lecture. 



What are the key applications of the MacWilliams identities in coding theory?
There is a very nice proof that there is no projective plane with order 6 mod 8 in Assmus, E. F., Jr.; Maher, David P. Nonexistence proofs for projective designs. Amer. Math. Monthly 85 (1978), no. 2, 110–112. This uses the weight enumerator.


