Bourbaki dangerous bend symbol
Certains passages sont destinés à prémunir le lecteur contre des erreurs graves, où il risquerait de tomber; ces passages sont signalés en marge par le signe ☡ («tournant dangereux»)
Some passages are designed to forewarn the reader against serious errors, where he risks falling; these passages are signposted in the margin with the sign ☡ ("dangerous bend")
— Nicholas Bourbaki's description of the symbol in several textbooks[1]
The dangerous bend or caution symbol ☡ (unicode U+2621, "CAUTION SIGN") was created by the Nicolas Bourbaki group of mathematicians and appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument.[2]
Others have used variations of the symbol in their textbooks, and computer scientist Donald Knuth introduced a more realistic road-sign depiction in his Metafont and TeX systems, with a pair of adjacent signs indicating doubly dangerous passages.[3][4][5]
Typography
In HTML the unicode dangerous bend symbol ☡ can be produced by:
- ☡
In the LaTeX typesetting system, Knuth's dangerous bend symbol can be produced by first loading the font manfnt (a font with extra symbols used in Knuth's TeX manual) with
- \usepackage{manfnt}
and then typing
- \manfntsymbol{127}
References
- ^ See, for example, Théorie des ensembles, p. I-8.
- ^ Steven G. Krantz (2011), The Proof Is in the Pudding: The Changing Nature of Mathematical Proof, Springer, ISBN 0387489088, p. 92.
- ^ Donald Ervin Knuth (1984), The TeXbook, Addison-Wesley, ISBN 0201134489.
- ^ Donald Ervin Knuth (1986), The METAFONTbook, Addison-Wesley, ISBN 0201134454.
- ^ George J. Tourlakis (2003), Lectures in Logic and Set Theory, Volume 2: Set Theory, Cambridge University Press, ISBN 0521753740, p. xiv.