“Infinity symbol”的意思、由来-开放百科全书

The infinity symbol {{math|∞|size=150%}} (sometimes called the lemniscate) is a mathematical symbol representing the concept of infinity.

History

The shape of a sideways figure eight has a long pedigree; for instance, it appears in the cross of Saint Boniface, wrapped around the bars of a Latin cross.^[1] However, John Wallis is credited with introducing the infinity symbol with its mathematical meaning in 1655, in his De sectionibus conicis.^[1]^[2]^[3]^[4] Wallis did not explain his choice of this symbol, but it has been conjectured to be a variant form of a Roman numeral for 1,000 (originally CIƆ, also CƆ), which was sometimes used to mean "many", or of the Greek letter ω (omega), the last letter in the Greek alphabet.^[5]

Leonhard Euler used an open variant of the symbol^[6] in order to denote "absolutus infinitus". Euler freely performed various operations on infinity, such as taking its logarithm. This symbol is not used anymore, and is not encoded as a separate character in Unicode.

Usage

In mathematics, the infinity symbol is used more often to represent a potential infinity,^[1] rather than to represent an actually infinite quantity such as the ordinal numbers and cardinal numbers (which use other notations). For instance, in the mathematical notation for summations and limits such as

the infinity sign is conventionally interpreted as meaning that the variable grows arbitrarily large (towards infinity) rather than actually taking an infinite value.

The infinity symbol may also be used to represent a point at infinity, especially when there is only one such point under consideration. This usage includes, for instance,

the infinite point of a projective line,^[7] and the point added to a topological space

to form its one-point compactification

.^[8]

In areas other than mathematics, the infinity symbol may take on other related meanings; for instance, it has been used in bookbinding to indicate that a book is printed on acid-free paper and will therefore be long-lasting.^[9]

Modern symbolism

In modern mysticism, the infinity symbol has become identified with a variation of the ouroboros, an ancient image of a snake eating its own tail that has also come to symbolize the infinite, and the ouroboros is sometimes drawn in figure-eight form to reflect this identification, rather than in its more traditional circular form.^[10]

In the works of Vladimir Nabokov, including The Gift and Pale Fire, the figure-eight shape is used symbolically to refer to the Möbius strip and the infinite, for instance in these books' descriptions of the shapes of bicycle tire tracks and of the outlines of half-remembered people. The poem after which Pale Fire is entitled explicitly refers to "the miracle of the lemniscate".^[11]

Graphic design

The well known shape and meaning of the infinity symbol have made it a common typographic element of graphic design. For instance, the Métis flag, used by the Canadian Métis people in the early 19th century, is based around this symbol.^[12] In modern commerce, corporate logos featuring this symbol have been used by, among others, Room for PlayStation Portable, Microsoft Visual Studio, Fujitsu, Coursera, and CoorsTek.

Encoding

The symbol is encoded in Unicode at {{unichar|221E|infinity}} and in LaTeX as \\infty:

The Unicode set of symbols also includes several variant forms of the infinity symbol, that are less frequently available in fonts: {{unichar|29DC|INCOMPLETE INFINITY|html=|size=100%|note=ISOtech entity ⧜}}, {{unichar|29DD|TIE OVER INFINITY|html=|size=100%}} and {{unichar|29DE|INFINITY NEGATED WITH VERTICAL BAR|html=|size=100%}} in block Miscellaneous Mathematical Symbols-B.^[13] The acid-free paper symbol mentioned above is encoded separately as {{unichar|267E|PERMANENT PAPER SIGN|html=}}.

See also

References

1. ^¹²{{cite book|title=Cosmic Imagery: Key Images in the History of Science|first=John D.|last=Barrow|authorlink=John D. Barrow|publisher=W. W. Norton & Company|year=2008|isbn=9780393061772|contribution=Infinity: Where God Divides by Zero|pages=339–340|url=https://books.google.com/books?id=uRg6iN10JCIC&pg=PA339}}
2. ^{{cite book|url=https://archive.org/details/bub_gb_03M_AAAAcAAJ/page/n17|title=De Sectionibus Conicis, Nova Methodo Expositis, Tractatus|last=Wallis|first=John|publisher=|year=1655|isbn=|location=|pages=4|language=Latin|chapter=Pars Prima|author-link=John Wallis}}
3. ^{{cite book | last = Scott | first = Joseph Frederick | edition = 2 | isbn = 0-8284-0314-7 | page = 24 | publisher = American Mathematical Society | title = The mathematical work of John Wallis, D.D., F.R.S., (1616-1703) | url = https://books.google.com/books?id=XX9PKytw8g8C&pg=PA24 | year = 1981}}
4. ^{{citation | last = Martin-Löf | first = Per | author-link = Per Martin-Löf | contribution = Mathematics of infinity | doi = 10.1007/3-540-52335-9_54 | location = Berlin | mr = 1064143 | pages = 146–197 | publisher = Springer | series = Lecture Notes in Computer Science | title = COLOG-88 (Tallinn, 1988) | volume = 417 | year = 1990}}
5. ^{{cite book|title=A Brief History of Infinity: The Quest to Think the Unthinkable|last=Clegg|first=Brian|publisher=Robinson|year=2003|isbn=9781841196503|location=|pages=}}
6. ^See for instance Cor. 1 p. 174 in: Leonhard Euler. Variae observationes circa series infinitas. Commentarii academiae scientiarum Petropolitanae 9, 1744, pp. 160-188.
7. ^{{cite book|title=Algebraic Geometry: An Introduction|first=Daniel|last=Perrin|publisher=Springer|year=2007|isbn=9781848000568|page=28|url=https://books.google.com/books?id=Vn1yR9qPvlMC&pg=PA28}}
8. ^{{cite book|title= Infinite Dimensional Analysis: A Hitchhiker's Guide|first1=Charalambos D.|last1=Aliprantis|first2=Kim C.|last2=Border|edition=3rd|publisher=Springer|year=2006|isbn=9783540295877|pages=56–57|url=https://books.google.com/books?id=4vyXtR3vUhoC&pg=PA56}}
9. ^{{cite book|title=A handbook for the study of book history in the United States|last1=Zboray|first1=Ronald J.|last2=Zboray|first2=Mary Saracino|publisher=Center for the Book, Library of Congress|year=2000|isbn=9780844410159|location=|page=49|pages=}}
10. ^{{cite book|url=https://books.google.com/books?id=vhNNrX3bmo4C&pg=PA243|title=Dreams, Illusion, and Other Realities|last=O'Flaherty|first=Wendy Doniger|publisher=University of Chicago Press|year=1986|isbn=9780226618555|page=243}} The book also features this image on its cover.
11. ^{{cite book|title=Nabokov: The Mystery of Literary Structures|first=Leona|last=Toker|publisher=Cornell University Press|year=1989|isbn=9780801422119|page=159|url=https://books.google.com/books?id=Jud1q_NrqpcC&pg=PA159}}
12. ^{{cite book|title=Native American Flags|first1=Donald T.|last1=Healy|first2=Peter J.|last2=Orenski|publisher=University of Oklahoma Press|year=2003|isbn=9780806135564|page=284}}
13. ^{{cite web|url=https://www.unicode.org/charts/PDF/U2980.pdf|title=Miscellaneous Mathematical Symbols-B|last=|first=|date=|website=|publisher=Unicode Consortium|archive-url=https://web.archive.org/web/20181112231107/https://www.unicode.org/charts/PDF/U2980.pdf|archive-date=12 November 2018|dead-url=No|accessdate=1 December 2013}}