Abstract for Donald E. Knuth, The Sandwich Theorem
This report contains expository notes about a function
$theta(G)$ that is popularly known as the Lov\'asz number of a
graph $G$. There are many ways to define $theta(G)$, and the
surprising variety of different characterizations indicates in
itself that $theta(G)$ should be interesting. But the most
interesting property of $theta(G)$ is probably the fact that
it can be computed efficiently, although it lies ``sandwiched''
between other classic graph numbers whose computation is NP-hard.
I have tried to make these notes self-contained so that they might
serve as an elementary introduction to the growing literature
on Lov\'asz's fascinating function.