In today’s lecture, we proved that Reed-Solomon codes meet the Singleton bound (and hence are *Maximum Distance Separable* or MDS). We also proved the following property of an MDS code with (intergal) dimension : For any subset of indices of size exactly , the code projected onto is the “trivial” code of distance one: .

The proof of the distance of RS codes can be found in the fall 07 notes on lecture 12.

In the second part of the lecture, we started to talk about 2-party communication complexity. In particular, I encourage you to think of protocols with low communication complexity for the following functions (in all the cases below ):

- if and only if (the sums are over ),
- if and only if (where denotes the Hamming weight of and is an integer),
- if and only if (where we think of and as integers),
- if and only if for every ,
- if and only if for every .

The functions and are known as (set) equality and (set) disjointness in the literature. (The connections to sets comes from the observation that any vector in can be thought of as the “characteristic vector” of a subset of .

Communication complexity is mostly used a lower bound tool in complexity. For more details, you can take a look at the book with the same name. On Wednesday, we will study some upper bounds on the communication complexity that uses Reed-Solomon codes.

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