TITLE: Ron L. Graham’s lesser known significant contributions
QUESTION [47 upvotes]: Ron L. Graham is sadly no longer with us.
He was very prolific and his work spanned many areas of mathematics including graph theory, computational geometry, Ramsey theory, and quasi-randomness. His long association with Paul Erdős is of course very well known. Graham’s number, and Graham-Rothschild theorem, and the wonderful book Concrete Mathematics are other well known contributions.
However, some of his contributions may not be as widely known, but deserve to be so. This question is to encourage people to comment on such contributions. I am not familiar with his work on scheduling theory, for example.
He was into magic tricks and the mathematics behind them and co-authored a book on this with Persi Diaconis. And he was into juggling, like Claude Shannon.
Edit: Thanks to @LSpice for pointing out the Meta MathOverflow thread here on personal anecdotes.

REPLY [5 votes]: Corollary (Graham). A rational number $p/q$ can be written as a sum of finitely many distinct reciprocals of integer squares iff $p/q \in [0,-1+\pi^2/6)~ \cup ~[1,\pi^2/6)$.
For the statement of the full theorem from which this follows, see On Finite Sums of Unit Fractions, with Graham as the sole author. Link.
This result is not very significant, but in literature surround unit/Egyptian fraction-esque problems, this result is cited often; if not for its use than for its novelty.