View source for Cis (mathematics)

{{Short description|Alternate mathematical notation for cos x + i sin x}}
{{Lowercase title}}
{{Redir|Cisoidal oscillation|the plane curve generated from two curves|Cissoid}}
{{Use dmy dates|date=July 2019|cs1-dates=y}}
{{math|'''cis'''}}<!-- with a lower-case "c" --> is a [[mathematical notation]] defined by {{math|1=cis ''x'' = cos ''x'' + ''i'' sin ''x''}},<ref group="nb" name="NB_ij"/> where {{math|cos}} is the [[cosine]] function, {{mvar|i}} is the [[imaginary unit]] and {{math|sin}} is the [[sine]] function. {{mvar|x}} is the [[Argument (complex analysis)|argument]] of the complex number (angle between line to point and x-axis in [[polar form]]). The notation is less commonly used in mathematics than [[Euler's formula]], {{nowrap|{{math|''e<sup>ix</sup>''}},}} which offers an even shorter notation for {{nowrap|{{math|cos ''x'' + ''i'' sin ''x''}},}} but <code>cis(x)</code> is widely used as a name for this function in [[Library (computing)|software libraries]].

== Overview ==
The {{math|cis}} notation is a shorthand for the combination of functions on the right-hand side of [[Euler's formula]]:
: <math>e^{ix} = \cos x + i\sin x,</math>
where {{math|''i''<sup>2</sup> {{=}} −1}}. So,
: <math>\operatorname{cis} x = \cos x + i\sin x,</math><ref name="Weisstein_cis"/><ref name="Simmons_2014_1"/><ref name="Rationale_2003_C"/><ref name="Amann-Escher_2006"/>
i.e. "{{math|cis}}" is an [[acronym]] for "{{math|Cos ''i'' Sin}}".

It connects [[trigonometric function]]s with [[exponential function]]s in the [[complex plane]] via Euler's formula. While the [[domain of definition]] is usually <math>x \in \mathbb{R}</math>, [[complex value]]s <math>z \in \mathbb{C}</math> are possible as well:
: <math>\operatorname{cis} z = \cos z + i\sin z,</math>
so the {{math|cis}} function can be used to extend Euler's formula to a more [[general complex exponential function|general complex version]].<ref name="Moskowitz_2002"/>

The function is mostly used as a convenient shorthand notation to simplify some expressions,<ref name="Swokowski_2011"/><ref name="Reis_2011"/><ref name="Weitz_2016"/> for example in conjunction with [[Fourier transform|Fourier]] and [[Hartley transform]]s,<ref name="Byrnes_2004"/><ref name="Kammler_2008"/><ref name="Lorenzo-Hartley_2016"/> or when exponential functions shouldn't be used for some reason in math education.

In information technology, the function sees dedicated support in various high-performance math libraries (such as [[Intel]]'s [[Math Kernel Library]] (MKL)<ref name="Intel_MKL"/> or MathCW<ref name="Beebe_2017"/>), available for many compilers and programming languages (including [[C (programming language)|C]], [[C++]],<ref name="Intel_2007_C++"/> [[Common Lisp]],<ref name="Lisp"/><ref name="Lisp_2005"/> [[D (programming language)|D]],<ref name="D_2011"/> [[Haskell (programming language)|Haskell]],<ref name="Haskell"/> [[Julia (programming language)|Julia]],<ref name="Julia"/> and [[Rust (programming language)|Rust]]<ref name="Rust"/>). Depending on the platform, the [[fused operation]] is about twice as fast as calling the sine and cosine functions individually.<ref name="D_2011"/><ref name="Rationale_2003_C"/>

== Mathematical identities ==
=== Derivative ===
: <math>\frac{\mathrm{d}}{\mathrm{d}z}\operatorname{cis} z = i\operatorname{cis} z = ie^{iz}</math><ref name="Weisstein_cis"/><ref name="Fuchs_2011_2"/>

=== Integral ===
: <math>\int\operatorname{cis} z \,\mathrm{d}z = -i\operatorname{cis} z = -ie^{iz}</math><ref name="Weisstein_cis"/>

=== Other properties ===
These follow directly from [[Euler's formula]].
: <math>\cos(x) = \frac{\operatorname{cis}(x) + \operatorname{cis}(-x)}{2} = \frac{e^{ix} + e^{-ix}}{2}</math>
: <math>\sin(x) = \frac{\operatorname{cis}(x) - \operatorname{cis}(-x)}{2i}= \frac{e^{ix} - e^{-ix}}{2i}</math>
: <math>\operatorname{cis}(x+y) = \operatorname{cis} x\,\operatorname{cis} y</math><ref name="Fuchs_2011_1"/>
: <math>\operatorname{cis}(x-y) = {\operatorname{cis} x \over \operatorname{cis} y}</math>

The identities above hold if {{mvar|x}} and {{mvar|y}} are any complex numbers. If {{mvar|x}} and {{mvar|y}} are real, then
: <math>|\operatorname{cis} x - \operatorname{cis} y| \le |x-y|.</math><ref name="Fuchs_2011_1"/>

== History ==
The {{math|cis}} notation was first coined by [[William Rowan Hamilton]] in ''Elements of Quaternions'' (1866)<ref name="Hamilton_1866"/><ref name="Hamilton_1899"/> and subsequently used by [[Irving Stringham]] (who also called it "[[circular sector|sector]] of {{math|''x''}}") in works such as ''Uniplanar Algebra'' (1893),<ref name="Stringham_1893"/><ref name="Cajori_1929"/> [[James Harkness (mathematician)|James Harkness]] and [[Frank Morley]] in their ''Introduction to the Theory of Analytic Functions'' (1898),<ref name="Cajori_1929"/><ref name="Harkness_Morley_1898"/> or by [[George Ashley Campbell]] (who also referred to it as "cisoidal oscillation") in his works on [[transmission line]]s (1901<!-- possibly earlier -->) and [[Fourier integral]]s (1928).<ref name="Campbell_1901"/><ref name="Campbell_1911"/><ref name="Campbell_1928"/>

{{anchor|cas}}In 1942, inspired by the {{math|cis}} notation, [[Ralph V. L. Hartley]] introduced the {{math|[[cas (mathematics)|cas]]}} (for ''cosine-and-sine'') function for the real-valued [[Hartley kernel]], a meanwhile established shortcut in conjunction with [[Hartley transform]]s:<ref name="Hartley_1942"/><ref name="Bracewell_1999"/>
: <math>\operatorname{cas} x = \cos x + \sin x.</math>

== Motivation ==
The {{math|cis}} notation is sometimes used to emphasize one method of viewing and dealing with a problem over another.<ref name="Diehl-Leupp_2010"/> The mathematics of trigonometry and exponentials are related but not exactly the same; exponential notation emphasizes the whole, whereas {{math|cis ''x''}} and {{math|cos ''x'' + ''i'' sin ''x''}} notations emphasize the parts. This can be rhetorically useful to mathematicians and engineers when discussing this function, and further serve as a [[mnemonic]] (for {{math|cos + ''i'' sin}}).<ref name="Campbell_1928"/>

The {{math|cis}} notation is convenient for math students whose knowledge of trigonometry and complex numbers permit this notation, but whose conceptual understanding does not yet permit the notation {{math|''e<sup>ix</sup>''}}. The usual proof that {{math|1=cis ''x'' = ''e<sup>ix</sup>''}} requires [[calculus]], which the student may not have studied before encountering the expression {{math|cos ''x'' + ''i'' sin ''x''}}.

This notation was more common when typewriters were used to convey mathematical expressions.{{cn|date=September 2024}}

== See also ==
* [[De Moivre's formula]]
* [[Euler's formula]]
* [[Complex number]]
* [[Ptolemy's theorem]]
* [[Phasor]]
* [[Versor]]

== Notes ==
{{reflist|group="nb"|refs=
<ref group="nb" name="NB_ij">Here, {{mvar|i}} refers to the [[imaginary unit]] in [[mathematics]]. Since {{mvar|i}} is commonly used to denote [[electric current]] in [[electrical engineering]] and [[control systems engineering]], the imaginary unit is alternatively denoted by {{mvar|j}} instead of {{mvar|i}} in these contexts. Regardless of context, this does not affect the established name of the function as {{math|cis}}.</ref>
}}

== References ==
{{reflist|refs=
<ref name="Byrnes_2004">{{cite book |title=Computational Noncommutative Algebra and Applications |editor-first=Jim |editor-last=Byrnes |author-first1=Ekaterina |author-last1=L.-Rundblad |author-first2=Alexei |author-last2=Maidan |author-first3=Peter |author-last3=Novak |author-first4=Valeriy |author-last4=Labunets |chapter=Fast Color Wavelet-Haar-Hartley-Prometheus Transforms for Image Processing |series=NATO Science Series II: Mathematics, Physics and Chemistry (NAII) |volume=136 |publisher=[[Springer Science + Business Media, Inc.]] |location=Prometheus Inc., Newport, USA |publication-place=Dordrecht, Netherlands |date=2004 |isbn=978-1-4020-1982-1 |issn=1568-2609 |doi=10.1007/1-4020-2307-3 |pages=401–411 |url=http://web.cecs.pdx.edu/~mperkows/CAPSTONES/Quaternion/katya2.pdf |access-date=2017-10-28 |url-status=live |archive-url=https://web.archive.org/web/20171028130812/http://web.cecs.pdx.edu/~mperkows/CAPSTONES/Quaternion/katya2.pdf |archive-date=2017-10-28}}</ref>
<ref name="Simmons_2014_1">{{cite web |title=Cis |author-first=Bruce |author-last=Simmons |date=2014-07-28 |orig-date=2004 |work=Mathwords: Terms and Formulas from Algebra I to Calculus |publisher=[[Clackamas Community College]], Mathematics Department |location=Oregon City, Oregon, USA |url=http://www.mathwords.com/c/cis.htm |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716150005/http://www.mathwords.com/c/cis.htm |archive-date=2023-07-16}}</ref>
<ref name="Kammler_2008">{{cite book |title=A First Course in Fourier Analysis |author-first=David W. |author-last=Kammler |publisher=[[Cambridge University Press]] |date=2008-01-17 |edition=2 |isbn=978-1-13946903-6 |url=https://books.google.com/books?id=znP-ADtE8ZQC |access-date=2017-10-28}}</ref>
<ref name="Lorenzo-Hartley_2016">{{cite book |title=The Fractional Trigonometry: With Applications to Fractional Differential Equations and Science |author-first1=Carl F. |author-last1=Lorenzo |author-first2=Tom T. |author-last2=Hartley |publisher=[[John Wiley & Sons]] |date=2016-11-14 |isbn=978-1-11913942-3 |url=https://books.google.com/books?id=LdyADQAAQBAJ |access-date=2017-10-28}}</ref>
<ref name="Hamilton_1866">{{cite book |title=Elements of Quaternions |author-first=William Rowan |author-last=Hamilton |author-link=William Rowan Hamilton |date=1866-01-01 |edition=1 |editor-first=William Edwin |editor-last=Hamilton |editor-link=William Edwin Hamilton |publisher=[[Longmans, Green & Co.]], [[University Press (Dublin)|University Press]], [[Michael Henry Gill]] |publication-place=London, UK |location=Dublin, Irland |chapter=Book II, Chapter II. Fractional powers, General roots of unity |pages=250–257, 260, 262–263 |chapter-url=https://archive.org/details/elementsquaterni00hamirich/page/n323 |access-date=2016-01-17 |quote-pages=250, 252 |quote=[...] {{nowrap|cos [...] + ''i'' sin [...]}} we shall occasionally ''abridge'' to the following: [...] cis [...]. As to the marks [...], they are to be considered as chiefly available for the present ''exposition'' of the system, and as not often wanted, nor employed, in the subsequent ''practise'' thereof; and the same remark applies to the recent ''abrigdement'' cis, for {{nowrap|cos + ''i'' sin}} [...]}} ([https://archive.org/stream/elementsquaterni00hamirich#page/n0/mode/1up], [https://archive.org/details/elementsquatern02hamigoog][https://books.google.com/books?id=b2stAAAAYAAJ]) (NB. This work was published posthumously, Hamilton died in 1865.)</ref>
<ref name="Hamilton_1899">{{cite book |title=Elements of Quaternions |author-first=William Rowan |author-last=Hamilton |author-link=William Rowan Hamilton |editor-first1=William Edwin |editor-last1=Hamilton |editor-link1=William Edwin Hamilton |editor-first2=Charles Jasper |editor-last2=Joly |editor-link2=Charles Jasper Joly |date=1899 |orig-date=1866-01-01 |edition=2 |volume=I |publisher=[[Longmans, Green & Co.]] |location=London, UK |page=262 |url=https://archive.org/details/117770258_001 |access-date=2019-08-03 |quote-page=262 |quote=[...] recent ''abridgment'' cis for {{nowrap|cos + ''i'' sin}} [...]}} (NB. This edition was reprinted by [[Chelsea Publishing Company]] in 1969.)</ref>
<ref name="Stringham_1893">{{anchor|Stringham-1893}}{{cite book |title=Uniplanar Algebra, being part&nbsp;1 of a propædeutic to the higher mathematical analysis |author-first=Irving |author-last=Stringham |author-link=Irving Stringham |date=1893-07-01 |orig-date=1891 |edition=1 |volume=1 |publisher=[[The Berkeley Press]] |others=C. A. Mordock & Co. (printer) |location=San Francisco, California, USA |pages=71–75, 77, 79–80, 82, 84–86, 89, 91–92, 94–95, 100–102, 116, 123, 128–129, 134–135 |url=https://archive.org/details/uniplanaralgebra00stri |access-date=2016-01-18 |quote-page=71 |quote=As an abbreviation for {{nowrap|cos ''θ'' + ''i'' sin ''θ''}} it is convenient to use cis&nbsp;''θ'', which may be read: ''sector of θ''.}}</ref>
<ref name="Cajori_1929">{{cite book |author-first=Florian |author-last=Cajori |author-link=Florian Cajori |title=A History of Mathematical Notations |volume=2 |orig-date=March 1929 |publisher=[[Open court publishing company]] |location=Chicago, Illinois, USA |date=1952 |edition=3rd corrected printing of 1929 issue, 2nd |page=133 |isbn=978-1-60206-714-1 |url=https://books.google.com/books?id=bT5suOONXlgC |access-date=2016-01-18 |quote-page=133 |quote=[[#Stringham-1893|Stringham]] denoted {{nowrap|cos ''β'' + ''i'' sin ''β''}} by "cis&nbsp;''β''", a notation also used by [[#Harkness-Morley-1898|Harkness and Morley]].}} (NB. ISBN and link for reprint of 2nd edition by Cosimo, Inc., New York, US, 2013.)</ref>
<ref name="Harkness_Morley_1898">{{anchor|Harkness-Morley-1898}}{{cite book |title=Introduction to the Theory of Analytic Functions |author-first1=James |author-last1=Harkness |author-link1=James Harkness (mathematician) |author-first2=Frank |author-last2=Morley |author-link2=Frank Morley |date=1898 |edition=1 |publisher=[[Macmillan and Company]] |location=London, UK |pages=[https://archive.org/details/cu31924059412910/page/n234 18], 22, 48, 52, 170 |url=https://archive.org/details/cu31924059412910 |access-date=2016-01-18 |isbn=978-1-16407019-1}} (NB. ISBN for reprint by [[Kessinger Publishing]], 2010.)</ref>
<ref name="Swokowski_2011">{{cite book |title=Precalculus: Functions and Graphs |series=Precalculus Series |author-first1=Earl William |author-last1=Swokowski |author-link1=:d:Q59629142 |author-first2=Jeffery |author-last2=Cole |edition=12 |publisher=[[Cengage Learning]] |date=2011 |isbn=978-0-84006857-6 |url=https://books.google.com/books?id=8GB2Udf8wnoC |access-date=2016-01-18}}</ref>
<ref name="Intel_MKL">{{cite book |title=Developer Reference for Intel Math Kernel Library (Intel MKL) 2017 - C |series=MKL documentation; IDZ Documentation Library |id=671504 |date=2016-09-06 |chapter=v?CIS |publisher=[[Intel Corporation]] |page=1799 |url=https://www.intel.com/content/www/us/en/content-details/671504/developer-reference-for-intel-math-kernel-library-intel-mkl-2017-c.html |access-date=2016-01-15}}</ref>
<ref name="Intel_2007_C++">{{cite web |title=Intel C++ Compiler Reference |date=2007 |orig-date=1996 |publisher=[[Intel Corporation]] |pages=34, 59–60 |id=307777-004US |url=http://supercomputer.susu.ru/upload/users/instructions/C_Compiler_Reference.pdf |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716150413/http://supercomputer.susu.ru/upload/users/instructions/C_Compiler_Reference.pdf |archive-date=2023-07-16}}</ref>
<ref name="Lisp">{{cite web |title=CIS |work=[[Common Lisp Hyperspec]] |date=1996 |publisher=[[The Harlequin Group Limited]] |url=http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/fun_cis.html |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716150421/http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/fun_cis.html |archive-date=2023-07-16}}</ref>
<ref name="Lisp_2005">{{cite web |title=CIS |publisher=[[LispWorks, Ltd.]] |date=2005 |orig-date=1996 |url=http://clhs.lisp.se/Body/f_cis.htm#cis |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716150740/http://clhs.lisp.se/Body/f_cis.htm#cis |archive-date=2023-07-16}}</ref>
<ref name="D_2011">{{cite web |title=std.math: expi |work=D programming language |date=2016-01-11 |orig-date=2000 |publisher=[[Digital Mars]] |url=http://dlang.org/phobos/std_math.html#.expi |access-date=2016-01-14 |url-status=live |archive-url=https://web.archive.org/web/20230716150623/https://dlang.org/phobos/std_math.html#.expi |archive-date=2023-07-16}}</ref>
<ref name="Julia">{{cite web |url=https://docs.julialang.org/en/v1.0/base/math/#Base.cis |title=Mathematics; Mathematical Operators |work=The Julia Language |access-date=2019-12-05 |url-status=live |archive-url=https://web.archive.org/web/20200819172952/https://docs.julialang.org/en/v1.0/base/math/ |archive-date=2020-08-19}}</ref>
<ref name="Rust">{{cite web |title=Struct num_complex::Complex |url=https://docs.rs/num-complex/latest/num_complex/struct.Complex.html#method.cis |access-date=2022-08-05 |url-status=live |archive-url=https://web.archive.org/web/20230716150903/https://docs.rs/num-complex/latest/num_complex/struct.Complex.html#method.cis |archive-date=2023-07-16}}</ref>
<ref name="Haskell">{{cite web |title=CIS |work=Haskell reference |publisher=ZVON |url=http://zvon.org/other/haskell/Outputcomplex/cis_f.html |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716151029/http://zvon.org/other/haskell/Outputcomplex/cis_f.html |archive-date=2023-07-16}}</ref>
<ref name="Rationale_2003_C">{{cite web |title=Rationale for International Standard - Programming Languages - C |version=5.10 |date=April 2003 |pages=114, 117, 183, 186–187 |url=http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf |access-date=2010-10-17 |url-status=live |archive-url=https://web.archive.org/web/20160606072228/http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf |archive-date=2016-06-06}}</ref>
<ref name="Weisstein_cis">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Cis |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |orig-date=2000 |date=2015 |url=http://mathworld.wolfram.com/Cis.html |access-date=2016-01-09 |url-status=live |archive-url=https://web.archive.org/web/20160127061403/http://mathworld.wolfram.com/Cis.html |archive-date=2016-01-27}}</ref>
<ref name="Fuchs_2011_2">{{cite book |title=Analysis I |language=German |author-first=Martin |author-last=Fuchs |date=2011 |publisher=Fachrichtung 6.1 Mathematik, [[Universität des Saarlandes]], Germany´|chapter=Chapter 11: Differenzierbarkeit von Funktionen |edition=WS<!-- Wintersemester --> 2011/2012 |pages=3, 13 |url=http://www.math.uni-sb.de/ag/fuchs/Ana1/Paragraph11.pdf |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716161241/https://www.math.uni-sb.de/ag/fuchs/Ana1/Paragraph11.pdf |archive-date=2023-07-16}}</ref>
<ref name="Fuchs_2011_1">{{cite book |title=Analysis I |language=German |author-first=Martin |author-last=Fuchs |date=2011 |publisher=Fachrichtung 6.1 Mathematik, [[Universität des Saarlandes]], Germany |chapter=Chapter 8.IV: Spezielle Funktionen – Die trigonometrischen Funktionen |edition=WS 2011/2012 |pages=16–20 |url=http://www.math.uni-sb.de/ag/fuchs/Ana1/Paragraph8.pdf |access-date=2016-01-15 |url-status=live |archive-url=https://web.archive.org/web/20230716151357/https://www.math.uni-sb.de/ag/fuchs/Ana1/Paragraph8.pdf |archive-date=2023-07-16}}</ref>
<ref name="Hartley_1942">{{cite journal |title=A More Symmetrical Fourier Analysis Applied to Transmission Problems |author-link=Ralph Hartley |author-last=Hartley |author-first=Ralph V. L. |journal=[[Proceedings of the IRE]] |publisher=[[Institute of Radio Engineers]] |volume=30 |issue=3 |date=March 1942 |doi=10.1109/JRPROC.1942.234333 |s2cid=51644127 |pages=144–150 |url=https://www.researchgate.net/publication/3468825 |access-date=2023-07-16 |url-status=live |archive-url=https://web.archive.org/web/20190405073552/https://www.researchgate.net/publication/3468825_A_More_Symmetrical_Fourier_Analysis_Applied_to_Transmission_Problems |archive-date=2019-04-05}}</ref>
<ref name="Bracewell_1999">{{cite book |author-link=Ronald N. Bracewell |author-last=Bracewell |author-first=Ronald N. |title=The Fourier Transform and Its Applications |publisher=[[McGraw-Hill]] |edition=3 |orig-date=1985, 1978, 1965 |date=June 1999 |isbn=978-0-07303938-1}}</ref>
<ref name="Reis_2011">{{cite book |title=Abstract Algebra: An Introduction to Groups, Rings and Fields |author-first=Clive |author-last=Reis |date=2011 |edition=1 |publisher=[[World Scientific Publishing Co. Pte. Ltd.]] |isbn=978-9-81433564-5 |pages=434–438}}</ref>
<ref name="Weitz_2016">{{cite web |title=The fundamental theorem of algebra - a visual proof |author-first=Edmund |author-last=Weitz |author-link=:de:Edmund Weitz |date=2016 |publisher=[[Hamburg University of Applied Sciences]] (HAW), Department Medientechnik |location=Hamburg, Germany |url=http://weitz.de/fund/ |access-date=2019-08-03 |url-status=live |archive-url=https://archive.today/20190803122756/http://weitz.de/fund/ |archive-date=2019-08-03}}</ref>
<ref name="Diehl-Leupp_2010">{{cite book |title=Komplexe Zahlen: Ein Leitprogramm in Mathematik |language=de |author-first1=Christina |author-last1=Diehl |author-first2=Marcel |author-last2=Leupp |location=Basel & Herisau, Switzerland |publisher=[[Eidgenössische Technische Hochschule Zürich]] (ETH) |date=January 2010 |page=41 |url=https://ethz.ch/content/dam/ethz/special-interest/dual/educeth-dam/documents/Unterrichtsmaterialien/mathematik/Komplexe%20Zahlen%20(Leitprogramm)/Leitprogramm.pdf |url-status=live |archive-url=https://web.archive.org/web/20170827073841/https://www.ethz.ch/content/dam/ethz/special-interest/dual/educeth-dam/documents/Unterrichtsmaterialien/mathematik/Komplexe%20Zahlen%20(Leitprogramm)/Leitprogramm.pdf |archive-date=2017-08-27 |quote-page=41 |quote=[...] Bitte vergessen Sie aber nicht, dass e<sup>''iφ''</sup> für uns bisher nur eine Schreibweise für den Einheitszeiger mit Winkel ''φ'' ist. In anderen Büchern wird dafür oft der Ausdruck cis(''φ'') anstelle von e<sup>''iφ''</sup> verwendet. [...]}} (109 pages)</ref>
<ref name="Campbell_1901">{{cite journal |title=Chapter XXX. On loaded lines in telephonic transmission |author-link=George Ashley Campbell |author-first=George Ashley |author-last=Campbell |date=1903 |orig-date=1901-06-07 |volume=5 |issue=27 |journal=[[The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science]] |series=Series 6 |publisher=[[Taylor & Francis]] |doi=10.1080/14786440309462928 |pages=313–330 |url=https://jontalle.web.engr.illinois.edu/Public/Campbell-LoadedLines.03.pdf |access-date=2023-07-16 |url-status=live |archive-url=https://web.archive.org/web/20230716113406/https://jontalle.web.engr.illinois.edu/Public/Campbell-LoadedLines.03.pdf |archive-date=2023-07-16}} (2+18 pages)</ref>
<ref name="Campbell_1911">{{cite journal |title=Cisoidal oscillations |author-link=George Ashley Campbell |author-first=George Ashley |author-last=Campbell |journal=[[Proceedings of the American Institute of Electrical Engineers]] |publisher=[[American Institute of Electrical Engineers]] |volume=XXX |issue=1–6 |date=April 1911 |doi=10.1109/PAIEE.1911.6659711 |s2cid=51647814 |pages=789–824 |url=https://ia800708.us.archive.org/view_archive.php?archive=/28/items/crossref-pre-1923-scholarly-works/10.1109%252Fpaiee.1910.6660428.zip&file=10.1109%252Fpaiee.1911.6659711.pdf |access-date=2023-06-24}} (37 pages)</ref>
<ref name="Campbell_1928">{{cite journal |title=The Practical Application of the Fourier Integral |author-link=George Ashley Campbell |author-first=George Ashley |author-last=Campbell |journal=[[The Bell System Technical Journal]] |publisher=[[American Telephone and Telegraph Company]] |volume=7 |issue=4 |date=1928-10-01 |orig-date=1927-09-13 |doi=10.1002/j.1538-7305.1928.tb00347.x |s2cid=53552671 |pages=639–707 [641] |url=https://ia803204.us.archive.org/11/items/bstj7-4-639/bstj7-4-639_text.pdf |access-date=2023-06-24 |quote-page=641 |quote=It has been recognized, almost from the start, however, that the form which best combines mathematical simplicity and complete generality makes use of the exponential oscillating function ''e''<sup>''i''2π''ft''</sup>. More recently the overwhelming advantage of using this oscillating function in the discussion of sinusoidal oscillatory systems has been generally recognized. It is, therefore, plain that this oscillating function should be adopted as the basic oscillation for both of the proposed tables. A name for this oscillation, associating it with sines and cosines, rather than with the real exponential function, seems desirable. The abbreviation cis ''x'' for (cos ''x'' + ''i'' sin ''x'') suggests that we name this function a cis or a cisoidal oscillation.}} (69 pages)</ref>
<ref name="Amann-Escher_2006">{{cite book |title=Analysis I |language=de |author-first1=Herbert |author-last1=Amann |author-link1=:d:Q102078329 |author-first2=Joachim |author-last2=Escher |author-link2=:de:Joachim Escher (Mathematiker) |series=Grundstudium Mathematik |publisher=[[Birkhäuser Verlag]] |location=Basel, Switzerland |date=2006 |edition=3 |isbn=978-3-76437755-7 |id={{ISBN|3-76437755-0}} |pages=292, 298}} (445 pages)</ref>
<ref name="Beebe_2017">{{cite book |author-first=Nelson H. F. |author-last=Beebe |title=The Mathematical-Function Computation Handbook - Programming Using the MathCW Portable Software Library |chapter=Chapter 15.2. Complex absolute value |date=2017-08-22 |location=Salt Lake City, Utah, USA |publisher=[[Springer International Publishing AG]] |edition=1 |lccn=2017947446 |isbn=978-3-319-64109-6 |doi=10.1007/978-3-319-64110-2 |s2cid=30244721 |page=443}}</ref>
<ref name="Moskowitz_2002">{{cite book |title=A Course in Complex Analysis in One Variable |chapter=Chapter 1. First Concepts |author-first=Martin A. |author-last=Moskowitz<!-- possibly |author-link=:d:Q102185284 --> |publisher=[[World Scientific Publishing Co. Pte. Ltd.]] |location=City University of New York Graduate Center, New York, USA |publication-place=Singapore |date=2002
|isbn=981-02-4780-X |page=7 |chapter-url=https://books.google.com/books?id=Acw5DwAAQBAJ&pg=PA7}} (ix+149 pages)</ref>
}}

{{DEFAULTSORT:Cis}}
[[Category:Trigonometry]]
[[Category:Mathematical identities]]