texbook.tex contains a double-dangerous-bend exercise which starts at line 18198 :

\ddangerexercise ^{Powers of ten}: The whole \TeX\ language has now been
summarized completely. To~demonstrate how much you know, name all of the ways
you can think of in which the numbers 10, ^^{Derek, Bo} 100, 1000, 10000,
and 100000 have special significance to \TeX.

Why can you find the sequence ^^{Derek, Bo} within that snippet?

Why does texbook.tex, Appendix I. Index (line 26324) also hold an entry Derek, Bo, 293.?

If I get this right, Bo Derek is an American film and television actress, and the name "Bo Derek" seems not to occur at all in the text of the TeXbook, only in the index. Why?

Addendum: Section 7 - My sense of humor of "Donald Knuth Interview 2006" on GitHub provides some insights. There is also a video of this interview. ;-)

  • 10
    Wikipedia has Bo Derek [...] is an American film and television actress, film producer, and model perhaps best known for her breakthrough film role in the sex comedy 10.
    – moewe
    Nov 9 '19 at 17:53

It's a somewhat dated joke, referencing a film that she starred in called "10".



Traditionally, the index in a mathematics text will contain a few funny or twisted or self referential entries.

This is one such example. Index entry "Bo Derek" points to "10" in the text. Those words appear only in the index, not in the text (but they are, of course in the TeX source). The connection is the movie referred to in @RicharSullivan 's answer.

There are other instance that should(?) have been indexed - for example

\exercise Statistics show that only 7.43 of 10 people who read this manual
actually type the |story.tex| file as recommended, but that those people
learn \TeX\ best. So why don't you join them?
\answer Laziness and/or obstinacy.

(Don't even think of trying to index all the references to 10pt .)

A google search for humorous index entry mathematics books found this from reddit:

Not a dedication, but I've seen an index entry to Gian-Carlo Rota that points to the index itself; he had no other references in the book. I think it was a real analysis text book.

One of my professors pointed it out, and the way he said it, it sounded like it was common knowledge. "Has everyone seen this by now?"

Does anyone know if this was a common thing among math text books?


Knuth is well known for these kinds of shenanigans.

I can't seem to document this tradition further, nor remember any examples (other than those I put in a book of my own). Perhaps visitors here can supply some (edit, or comment).

Edit. @FrankMittelbach 's comment led me to this from an interview with Knuth at https://github.com/kragen/knuth-interview-2006:

... there are lots of corny jokes in the indexes to my books now that people probably haven’t discovered yet, but somebody will ask me, why do I have a reference to Bo Derek in The TeXbook? And it turns out that just all the pages which are cited in the index for Bo Derek is where I used the number ten, so all the way through I’ve had this silly streak of some sort that means I don’t take everything too seriously.

There is just one instance of the string "Bo Derek" in the TeXbook TeX source, and that does refer to a page containing "10". I'm not sure whether it's correct to read the "just" in the quote from Knuth as claiming (falsely) that all the pages where "10" appears are indexed.

  • 1
    Underwood Dudley in "Elementary Number Theory" (archive.org/details/ElementaryNumberTheory) has an index entry for "sex in number theory".
    – Scott H.
    Nov 10 '19 at 3:24
  • 2
    There is a topology textbook that has an index entry for some professor's name point to a page where he is not mentioned at all. However, that page has an illustration of a sphere with toroidal handles and holes ... that remotely resembles a face Nov 10 '19 at 3:33
  • 4
    The original Sun Microsystems books on Java (published in the 1990s) had the spoof index entries, "Perfect numbers: 6, 28, 496" and "Prime numbers: 2, 3, 5, 7, 11, etc". And the textbook "Techniques of Finite Elements" by Irons and Ahmad had a drawing of a nasty looking bug labeled "Cimex fortranarius: magnification 10^16".
    – alephzero
    Nov 10 '19 at 11:36
  • 1
    and since Don in his interview claims that all number "10"s in the TeX book are indexed with Bo Derek you should be due a virtual $2.56 from him if you report it Nov 15 '19 at 8:30
  • @FrankMittelbach See my edit. I think Knuth stops just short of claiming to have indexed all the "10"s. If I'm wrong and therefore Knuth is wrong (in an informal transcript of an informal interview) can I claim a reward? Nov 15 '19 at 21:08

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