# A reward check from Donald Knuth

The other day I went to my pigeon-hole to collect my snail mail, and I had a letter from Donald E. Knuth, Professor Emeritus of the Art of Computer Programming!

Inside, there was a check for a correction I sent him some months ago. In fact, it was not really a correction; it was more like a comment. And it was so obvious (he even said that) that he just sent $\$$0.32, instead of the usual \$$2.56. But hey, who cares? I’ve got Knuth’s autograph now Perhaps I should set as one of my goals to find a proper error, so that I can receive a$\2.56 check By the way, the errata of the Concrete Mathematics is available online and this particular omission is documented as follows:

page 338, line 2 from the bottom
change “for $z$” to “for $z$ and multiplying by $a$”

## 6 thoughts on “A reward check from Donald Knuth”

1. That is great! I received on a few years back for providing an example of how a multi-way B-Tree could split during a delete! His response included the “sorry it took so long to get back to you – I have been busy on LaTex” (It was 20 years? Good thing I had not moved in that time frame!) It went on to say that he had taken notes and was returning the original note (along with the check). I never got back to see if I received acknowledgement in a subsequent edition, or even a problem for the issue.
Splitting can occur when you get too many entries in an internal node. Also, if you allow variable-length elements, a deletion and rearrangement of elements can cause overflow in the list of elements rather than the number of elements, which forces a node split.

2. Dear Mark,

a quick browsing shows that on page 485 of Volume 3, Knuth writes “Deletion from B-trees is only slightly more complicated than insertion (see exercise 6)”. The exercise 6 is to design an deletion algorithm for multiway B-trees, and the solution doesn’t mention anything about splitting.

Anyway, multiway B-trees are unknown to me, but I’ll definitely read Knuth’s section on them now. If I find any reference to you, I’ll let you know!

Joao

3. João,
I suggest modification on the photo, so that information useful for criminals is not displayed.
Abraços e parabéns pelo cheque.
Renato

4. Dear Renato,

thanks a lot for the comment! I’ve updated the image as suggested. It never occurred to me that the cheque info. could be used for criminal purposes.