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I searched the Comprehensive Latex Symbol list but couldn't find the symbol below. It's on the very last page of Knuth's Selected Papers on Fun & Games, which might mean it's a puzzle or mystery to go along with the rest of the puzzles in the book (pp. 741 in first printing; pp. 742 in second printing). I don't necessarily want to have the puzzle ruined for myself or others, but I've read the book cover to cover and searched high and low on the internet, and I can't really find any clues on how to proceed in finding more information about it. It's listed in the index as a reference to itself. Any hints?

EDIT: The index can be downloaded here: http://web.stanford.edu/group/cslipublications/cslipublications/site/9781575865843-knuth-fg-index.pdf

In the preface (pp. xiv), Knuth writes:

In conclusion, let me close this preface by warmly thanking the KTG society and its fictious leader, whose existence is sort of an enigma that can be deciphered only be carefully pondering the index at the rear of this book. Readers who are able to solve this weird puzzle will find that my eccentric muse was not at all able to resist the temptation to finish off the final preface of this eight-book series by adding some symbolic "book-ends," so that the fun and games would last longer.


Mystery symbol

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    I see an "et..." (aka old-style ampersand) followed by other stuff. – Steven B. Segletes Oct 4 '19 at 0:59
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    This is a letter invented by Dr. Seuss for his book On Beyond Zebra! It has no name, some call it ABCDEFGHIJKLMNOPQRSTUVWXYZ. See en.wikipedia.org/wiki/On_Beyond_Zebra! and the link to the Unicode extension on this page. The letter has nothing to do with the puzzle stated in the last paragraph of the preface. Knuth does not state that the last entry of the index gives a hint; you have to look up a different entry. And you need the book to see what is meant; the text above does not help. (And the puzzle has a partner that you have to find without any hint.) – Udo Wermuth Oct 4 '19 at 11:29
  • Does my previous comment contain the answer to your question? I write an answer that you can accept if it is the solution. And did you manage to solve the puzzle of the preface's last paragraph now that you do not concentrate only on the above symbol? The symbol is a joke that is not easy to understand for people who do not know the book. But as you wrote it is not fair to state the answer to the puzzle directly. – Udo Wermuth Oct 6 '19 at 9:28
  • @UdoWermuth Yes, thank you very much—your identification of the symbol was very helpful, as was your clarification of the puzzle of the preface's last paragraph. The last question I have is just how to type or draw the symbol in TeX. I found that it is in the Unicode registry here, but I am unsure how to access it. I haven't started on the other index puzzle, but I am happy to complete that on my own. – Academc Oct 6 '19 at 22:03
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This is a letter invented by Dr. Seuss for his children's book On Beyond Zebra! The author presents several letters which extend the known 26-letter alphabet of English.

In the book the letter is not assigned a name, but some call it ABCDEFGHIJKLMNOPQRSTUVWXYZ. See en.wikipedia.org/wiki/On_Beyond_Zebra! and the link to the registered Unicode extension on this page where it has the above stated name.

To get the symbol there are several ways but note if you want to treat the symbol as a letter its shape must match the other glyphs; you know a calligraphic A looks different from a sans-serif A, for example. Otherwise the symbol remains a symbol.

1) Find a Unicode font for which the font designer has created the letter and use a Unicode-aware TeX engine. (I don't know if such a font exists.)

2) Keep the symbol as an image and place it as a figure in your text. (I assume that the symbol is not used very often inside a text.)

3) Use, for example, Metafont to create the symbol yourself. (I think Knuth has done that as there are minor differences between the image above and the scanned image from the book; look, for example, at the top of the mirrored `B' at the right end.)

4) Try to approximate the symbol as it is done in the list of the Wikipedia page; there the symbol is composed of existing letters. (A reader must only be able to identify the letter; the letter has not a predefined shape as explained above.)

The letter has nothing to do with the puzzle stated in the last paragraph of the preface. Knuth does not state that the last entry of the index gives a hint; you have to look up a different entry. And you need the book to see what is meant; the text above does not help. (And the puzzle has a partner that you have to find without any hint.)

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    Thanks @UdoWermuth! Extremely helpful. Question: given point 3, which I'd independently noticed after having gone out to buy On Beyond Zebra!, would Knuth's reference to it in his texts be considered a typo or error? In my excitement at having noticed it, I emailed an error report to Knuth to that effect yesterday (and in my technical illiteracy as a humanities professor embarrassingly overlooked his reporting instructions... oops), but your 3rd paragraph about the distinction between a symbol and a letter has me doubting my report's legitimacy, even though Dr. Seuss's version is 'serifed' – Academc Oct 8 '19 at 10:16
  • (My interest in typos fits into my research on alphabets, so your distinction between symbol and letter was insightful. The rest of the Dr. Seuss 'alphabet' in On Beyond Zebra! is distinctly sans-serif, which fits Knuth's representation, but the ABCDEFGHIJKLMNOPQRSTUVWXYZ character has the 'serif'. If we assume Dr. Seuss created that original symbol as a letter containing the entire alphabet, Knuth's use of it is either a typo/font, depending. Not sure how to report that—but you're free to claim the reward check if you can explain it! I'm more interested in what it means for phil. of lang.) – Academc Oct 8 '19 at 10:30
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    @Academc I don't see any error that should be reported to Knuth. Letters exist as abstract ideas and the font designer creates an incarnation of the letters when he creates a font; a letter becomes a glyph (the word sort was used in the days of metal typesetting). For example, A is identified by a reader as the first letter of the alphabet. But if the bar is not vertical but has a slope it is still recognized as this letter. So in the idea of A the bar must not be vertical although the presented glyph has such a bar. Other interpretations might have the bar not touch both lines, – Udo Wermuth Oct 9 '19 at 16:43
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    omit the bar completely as the tip at the top still let us read the glyph as A, or uses an arc instead of a tip like the A in the Metafont logo. But two changes might destroy our ability to identify the letter: an arc and no bar looks more like N. Thus, Dr. Seuss describes a letter and has interpreted it in his handwriting. Knuth has interpreted the letter as a glyph, say, for the font Computer Modern Roman. The interpretations need not be identical. The abstract idea is something that we learn. If one is not familiar with, f.e., [Suetterlin] (en.wikipedia.org/wiki/Sütterlin) – Udo Wermuth Oct 9 '19 at 16:47
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    it gets difficult to identify the letter A or distinguish it from the letter G. And only a few people might have an abstract idea of the letters in On Beyond Zebra! – Udo Wermuth Oct 9 '19 at 16:48

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