[tex-k] minor errors in TeXbook Appendix G concerning genfrac

Sophie Alpert hi at sophiebits.com
Tue Dec 11 07:04:14 CET 2018

I believe I've found three minor errors in the TeXbook concerning
generalized fractions. I confirmed that none is corrected in
abcde-errata.pdf as posted on Donald Knuth's website ("Bugs in Computers &
Typesetting as of 15 Jan 2014").

1. TeXbook page A444 line 18 from the bottom mentions "let $(\lambda,
\rho)$ be the left and right delimiters". However it never uses those names
$\lambda$ and $\rho$ again, instead only mentioning the "left delimiter"
and "right delimiter" in step 15e on page A445. Either the Greek letters
are unnecessary and should be removed, or they are necessary and should be
used after their initial definition.

2. TeXbook page A445 line 13 says to replace "the generalized fraction by
an Inner atom...". I believe this text is meant to instruct replacing the
genfrac by an *Ord* atom. You can verify that in TeX a formula such as ${1
\over 2}.$ has no space between the fraction and the Punct, whereas it
would if the fraction became an Inner. (I'm not a WEB expert but I also see
no mention of Inner in the tex.web source, section "Put the \(f)fraction
into a box with its delimiters".)

3. Relatedly, TeXbook page A445 line 14 (end of Rule 15e) does not indicate
how to proceed after this processing. In contrast, Rules 1 through 14 all
end with an instruction to move to the next item or to jump to Rule 16 or
17. I believe this line should end with "and go to Rule 17". (Continuing to
Rule 16 would fix my second reported erratum, but it would not seem to make
sense to create an Inner atom only to immediately convert it to Ord.)

Sophie Alpert
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