Editing Weierstrass function

{{short description|Function that is continuous everywhere but differentiable nowhere}}
{{distinguish|text=the [[Weierstrass elliptic function]] (<math>\wp</math>) or the [[Weierstrass functions|Weierstrass sigma, zeta, or eta functions]]}}
{{Use dmy dates|date=September 2020}}
[[Image:WeierstrassFunction.svg|300px|thumb|Plot of Weierstrass function over the interval [&minus;2,&nbsp;2]. Like some other [[fractal]]s, the function exhibits [[self-similarity]]: every zoom (red circle) is similar to the global plot.]]

In [[mathematics]], the '''Weierstrass function''', named after its discoverer, [[Karl Weierstrass]], is an example of a real-valued [[function (mathematics)|function]] that is [[continuous function|continuous]] everywhere but [[Differentiable function|differentiable]] nowhere. It is also an example of a [[fractal curve]].

The Weierstrass function has historically served the role of a [[pathological (mathematics)|pathological]] function, being the first published example (1872) specifically concocted to challenge the notion that every continuous function is differentiable except on a set of isolated points.<ref>At least two researchers formulated continuous, nowhere differentiable functions before Weierstrass, but their findings were not published in their lifetimes.

Around 1831, [[Bernard Bolzano]] (1781–1848), a Czech mathematician, philosopher, and Catholic priest, constructed such a function; however, it was not published until 1922. See:

* Martin Jašek (1922) [http://dml.cz/bitstream/handle/10338.dmlcz/121916/CasPestMatFys_051-1922-2_2.pdf "Funkce Bolzanova"] (Bolzano's function), ''Časopis pro Pěstování Matematiky a Fyziky'' (Journal for the Cultivation of Mathematics and Physics), vol. 51, no. 2, pages 69–76 (in Czech and German).
* [[Vojtěch Jarník]] (1922) "O funkci Bolzanově" (On Bolzano's function), ''Časopis pro Pěstování Matematiky a Fyziky'' (Journal for the Cultivation of Mathematics and Physics), vol. 51, no. 4, pages 248 - 264 (in Czech).  Available on-line in Czech at:  http://dml.cz/bitstream/handle/10338.dmlcz/109021/CasPestMatFys_051-1922-4_5.pdf  .  Available on-line in English at:  http://dml.cz/bitstream/handle/10338.dmlcz/400073/Bolzano_15-1981-1_6.pdf  .
* Karel Rychlík (1923) "Über eine Funktion aus Bolzanos handschriftlichem Nachlasse" (On a function from Bolzano's literary remains in manuscript), ''Sitzungsberichte der königlichen Böhmischen Gesellschaft der Wissenschaften'' (Prag) (Proceedings of the Royal Bohemian Society of Philosophy in Prague) (for the years 1921-1922), Class II, no. 4, pages 1-20.  (''Sitzungsberichte'' was continued as:  ''Věstník Královské české společnosti nauk, třída matematicko-přírodovědecká'' (Journal of the Royal Czech Society of Science, Mathematics and Natural Sciences Class).)

Around 1860, Charles Cellérier (1818 - 1889), a professor of mathematics, mechanics, astronomy, and physical geography at the University of Geneva, Switzerland, independently formulated a continuous, nowhere differentiable function that closely resembles Weierstrass's function.  Cellérier's discovery was, however, published posthumously:

* Cellérier, C. (1890) [https://books.google.com/books?id=HMghAQAAIAAJ&pg=PA142 "Note sur les principes fondamentaux de l'analyse"] (Note on the fundamental principles of analysis), ''Bulletin des sciences mathématiques'', second series, vol. 14, pages 142 - 160.</ref> Weierstrass's demonstration that continuity did not imply almost-everywhere differentiability upended mathematics, overturning several proofs that relied on geometric intuition and vague definitions of [[smoothness]]. These types of functions were disliked by contemporaries: [[Charles Hermite]], on finding that one class of function he was working on had such a property, described it as a "lamentable scourge"{{Disputed inline|Accuracy of Poincaré quote in lead|date=March 2025}}.<ref>{{cite book |last1=Hermite |first1=Charles |author-link1=Charles Hermite |last2=Stieltjes |first2=Thomas |author-link2=Thomas Joannes Stieltjes |title=Correspondance d'Hermite et de Stieltjes |chapter=Letter 374, 20 May 1893 |editor-last1=Baillaud |editor-first1=Benjamin |editor-last2=Bourget |editor-first2=Henri |volume=2 |publisher=Gauthier-Villars |year=1905 |pages=317–319 |language=fr}}
</ref> The functions were difficult to visualize until the arrival of computers in the next century, and the results did not gain wide acceptance until practical applications such as models of [[Brownian motion]] necessitated infinitely jagged functions (nowadays known as fractal curves).<ref>{{cite web | url = https://nautil.us/maths-beautiful-monsters-234859/ | title = Math's Beautiful Monsters: How a destructive idea paved the way for modern math | last = Kucharski | first = Adam | date = 2017-10-26 | access-date = 2023-10-11}}</ref>

==Construction==

[[File:Weierstrass Animation.gif|thumb|right|Animation based on the increasing of the b value from 0.1 to 5.]]

In Weierstrass's original paper, the function was defined as a [[Fourier series]]:

<math display="block">f(x)=\sum_{n=0} ^\infty a^n \cos(b^n \pi x),</math>

where <math display="inline">0<a<1</math>, <math display="inline">b</math> is a positive odd integer, and

<math display="block"> ab > 1+\frac{3}{2} \pi.</math>

The minimum value of <math display="inline">b</math> for which there exists <math display="inline">0<a<1</math> such that these constraints are satisfied is <math display="inline">b=7</math>. This construction, along with the proof that the function is not differentiable at any point, was first delivered by Weierstrass in a paper presented to the [[Prussian Academy of Sciences|Königliche Akademie der Wissenschaften]] on 18 July 1872.<ref>On [https://books.google.com/books?id=G-YaAAAAYAAJ&pg=PA560 page 560] of the 1872 {{lang|de|Monatsberichte der Königlich Preußischen Akademie der Wissenschaften zu Berlin}} (Monthly Reports of the Royal Prussian Academy of Science in Berlin), there is a brief mention that on 18 July, {{lang|de|"Hr. Weierstrass las über stetige Funktionen ohne bestimmte Differentialquotienten"}} (Mr. Weierstrass read [a paper] about continuous functions without definite [i.e., well-defined] derivatives [to members of the Academy]). However, Weierstrass's paper was not published in the {{lang|de|Monatsberichte}}.</ref><ref>Karl Weierstrass, [https://books.google.com/books?id=1FhtAAAAMAAJ&pg=PA71 {{lang|de|italic=no|"Über continuirliche Functionen eines reellen Arguments, die für keinen Werth des letzeren einen bestimmten Differentialquotienten besitzen"}}] [On continuous functions of a real argument which possess a definite derivative for no value of the argument], in: {{lang|de|italic=no|Königlich Preußische Akademie der Wissenschaften}}, {{lang|de|italic=no|Mathematische Werke von Karl Weierstrass}} (Berlin, Germany:  Mayer & Mueller, 1895), vol. 2, pages 71–74.</ref><ref>See also: Karl Weierstrass, {{lang|de|Abhandlungen aus der Functionenlehre}} [''Treatises from the Theory of Functions''] (Berlin, Germany: Julius Springer, 1886), [https://books.google.com/books?id=fltYAAAAYAAJ&pg=PA97 page 97].</ref>

Despite being differentiable nowhere, the function is continuous: Since the terms of the infinite series which defines it are bounded by <math display="inline">\pm a^n</math> and this has finite sum for <math display="inline">0 < a < 1</math>, convergence of the sum of the terms is [[uniform convergence|uniform]] by the [[Weierstrass M-test]] with <math display="inline">M_n = a^n</math>. Since each partial sum is continuous, by the [[uniform limit theorem]], it follows that <math display="inline">f</math> is continuous. Additionally, since each partial sum is [[uniform continuity|uniformly continuous]], it follows that <math display="inline">f</math> is also uniformly continuous.

It might be expected that a continuous function must have a derivative, or that the set of points where it is not differentiable should be countably infinite or finite. According to Weierstrass in his paper, earlier mathematicians including [[Carl Friedrich Gauss|Gauss]] had often assumed that this was true. This might be because it is difficult to draw or visualise a continuous function whose set of nondifferentiable points is something other than a countable set of points. Analogous results for better behaved classes of continuous functions do exist, for example the [[Lipschitz functions]], whose set of non-differentiability points must be a [[Lebesgue null set]] ([[Rademacher's theorem]]). When we try to draw a general continuous function, we usually draw the graph of a function which is Lipschitz or otherwise well-behaved. Moreover, the fact that the set of non-differentiability points for a [[Monotonic function|monotone function]] is [[measure zero|measure-zero]] implies that the rapid oscillations of Weierstrass' function are necessary to ensure that it is nowhere-differentiable.

The Weierstrass function was one of the first [[fractals]] studied, although this term was not used until much later. The function has detail at every level, so zooming in on a piece of the curve does not show it getting progressively closer and closer to a straight line. Rather between any two points no matter how close, the function will not be monotone.

The computation of the [[Hausdorff dimension]] <math display="inline">D</math> of the graph of the classical Weierstrass function was an open problem until 2018, while it was generally believed that <math display="inline">D = 2 + \log_b(a) < 2</math>.<ref>Kenneth Falconer, ''The Geometry of Fractal Sets'' (Cambridge, England:  Cambridge University Press, 1985), pages 114, 149.</ref><ref>See also: Brian R. Hunt (1998), [http://www.ams.org/journals/proc/1998-126-03/S0002-9939-98-04387-1/S0002-9939-98-04387-1.pdf "The Hausdorff dimension of graphs of Weierstrass functions"], ''Proceedings of the American Mathematical Society'', vol. 126, no. 3, pages 791–800.</ref> That ''D'' is strictly less than 2 follows from the conditions on <math display="inline">a</math> and <math display="inline">b</math> from above. Only after more than 30 years was this proved rigorously.<ref>{{cite journal |author=Shen Weixiao |authorlink=Shen Weixiao |year=2018 |title=Hausdorff dimension of the graphs of the classical Weierstrass functions |journal=Mathematische Zeitschrift |issn=0025-5874 |doi=10.1007/s00209-017-1949-1 |arxiv=1505.03986 |s2cid=118844077 |volume=289 |issue=1–2 |pages=223–266}}</ref>

The term Weierstrass function is often used in [[real analysis]] to refer to any function with similar properties and construction to Weierstrass's original example. For example, the cosine function can be replaced in the infinite series by a [[Triangle wave|piecewise linear "zigzag" function]]. [[G. H. Hardy]] showed that the function of the above construction is nowhere differentiable with the assumptions <math display="inline">0 < a < 1, ab \geq 1</math>.<ref name="Hardy">Hardy G. H. (1916) "Weierstrass's nondifferentiable function", ''Transactions of the American Mathematical Society'', vol. 17, pages 301–325.</ref>

==Riemann function==
The Weierstrass function is based on the earlier Riemann function, claimed to be differentiable nowhere. Occasionally, this function has also been called the Weierstrass function.<ref>{{cite web|first1=Eric W.|last1=Weisstein|url=https://mathworld.wolfram.com/WeierstrassFunction.html|website=[[MathWorld]]|title=Weierstrass Function}}</ref>
<math display="block">
 f(x) = \sum_{n = 1}^\infty \frac{\sin(n^2x)}{n^2}.
</math>

While [[Bernhard Riemann]] strongly claimed that the function is differentiable nowhere, no evidence of this was published by Riemann, and Weierstrass noted that he did not find any evidence of it surviving either in Riemann's papers or orally from his students.

In 1916, [[G. H. Hardy]] confirmed that the function does not have a finite derivative in any value of <math display="inline">\pi x</math> where ''x'' is irrational or is rational with the form of either <math display="inline">\frac{2A}{4B+1}</math> or <math display="inline">\frac{2A+1}{2B}</math>, where ''A'' and ''B'' are integers.<ref name="Hardy"/> In 1969, [[Joseph Gerver]] found that the Riemann function has a defined differential on every value of ''x'' that can be expressed in the form of <math display="inline">\frac{2A+1}{2B+1}\pi</math> with integer ''A'' and ''B'', that is, rational multipliers of <math>\pi</math> with an odd numerator and denominator. On these points, the function has a derivative of <math display="inline">-\frac{1}{2}</math>.<ref>{{Cite journal |title=The Differentiability of the Riemann Function at Certain Rational Multiples of π |first=Joseph |last=Gerver |doi=10.1073/pnas.62.3.668 |journal=Proceedings of the National Academy of Sciences of the United States of America |date=1969 |volume=62 |issue=3 |pages=668–670 |doi-access=free |pmid=16591735 |pmc=223649|bibcode=1969PNAS...62..668G }}</ref> In 1971, J.&nbsp;Gerver showed that the function has no finite differential at the values of ''x'' that can be expressed in the form of <math display="inline">\frac{2A}{2B+1}\pi</math>, completing the problem of the differentiability of the Riemann function.<ref>{{cite journal |title=More on the Differentiability of the Riemann Function |first=Joseph |last=Gerver |doi=10.2307/2373445 |journal=American Journal of Mathematics |date=1971 |volume=93 |issue=1 |pages=33–41 |jstor=2373445 |s2cid=124562827}}</ref>

As the Riemann function is differentiable only on a [[null set]] of points, it is differentiable [[almost nowhere]].

==Hölder continuity==
It is convenient to write the Weierstrass function equivalently as

<math display="block">W_\alpha(x) = \sum_{n=0}^\infty b^{-n\alpha}\cos(b^n \pi x)</math>

for <math display="inline">\alpha = -\frac{\ln(a)}{\ln(b)}</math>.  Then <math display="inline">W_\alpha(x)</math> is [[Hölder continuous]] of exponent α, which is to say that there is a constant ''C'' such that

<math display="block">|W_\alpha(x)-W_\alpha(y)|\le C|x-y|^\alpha</math>

for all <math display="inline">x</math> and <math display="inline">y</math>.<ref>{{cite book | last1=Zygmund | first1=A. | title= Trigonometric Series |volume=I, II | title-link= Trigonometric Series | orig-year=1935 | publisher=[[Cambridge University Press]] | edition=3rd | series=Cambridge Mathematical Library | isbn=978-0-521-89053-3 | mr=1963498  | year=2002 |page=47}}</ref> Moreover, <math display="inline">W_1</math> is Hölder continuous of all orders <math display="inline">\alpha < 1</math> but not [[Lipschitz continuous]].

==Density of nowhere-differentiable functions==

It turns out that the Weierstrass function is far from being an isolated example: although it is "pathological", it is also "typical" of continuous functions:
* In a [[Topology|topological]] sense: the set of nowhere-differentiable real-valued functions on [0,&nbsp;1] is [[comeager set|comeager]] in the [[vector space]] ''C''([0,&nbsp;1];&nbsp;'''R''') of all continuous real-valued functions on [0,&nbsp;1] with the topology of [[uniform convergence]].<ref>{{cite journal|author=Mazurkiewicz, S..|title=Sur les fonctions non-dérivables|journal=Studia Math.|issue=3|year=1931|volume=3|pages=92–94|doi=10.4064/sm-3-1-92-94|doi-access=free}}</ref><ref>{{cite journal|author=Banach, S.|title=Über die Baire'sche Kategorie gewisser Funktionenmengen|journal=Studia Math.|issue=3|year=1931|volume=3|pages=174–179|doi=10.4064/sm-3-1-174-179|doi-access=free}}</ref>
* In a [[Measure theory|measure-theoretic]] sense: when the space ''C''([0,&nbsp;1];&nbsp;'''R''') is equipped with [[classical Wiener measure]] ''γ'', the collection of functions that are differentiable at even a single point of [0,&nbsp;1] has ''γ''-[[measure zero]]. The same is true even if one takes finite-dimensional "slices" of ''C''([0,&nbsp;1];&nbsp;'''R'''), in the sense that the nowhere-differentiable functions form a [[prevalent and shy sets|prevalent subset]] of ''C''([0,&nbsp;1];&nbsp;'''R''').

==See also==
*[[Blancmange curve]]
*[[Koch snowflake]]
*[[Nowhere continuous function]]

== Notes ==
{{Reflist}}

== References ==
*{{Citation | last=David | first=Claire  | year=2018 | title=Bypassing dynamical systems : A simple way to get the box-counting dimension of the graph of the Weierstrass function | journal=Proceedings of the International Geometry Center| publisher=Academy of Sciences of Ukraine | volume=11 | issue=2 | pages=53–68 | doi=10.15673/tmgc.v11i2.1028 | doi-access=free | arxiv=1711.10349 }}
*{{Citation | last=Falconer | first=K. | author-link=Kenneth Falconer (mathematician) | year=1984 | title=The Geometry of Fractal Sets | place=Cambridge | publisher=Cambridge University Press | series=Cambridge Tracts in Mathematics | volume=Book 85 | isbn=978-0-521-33705-2 | url={{Google books|-Kwp-GrimCIC|plainurl=yes}} }}
*{{Citation |last1=Gelbaum |first1=B Bernard R. |last2=Olmstead |first2=John M. H. |year=2003 |orig-year=1964 |title=Counterexamples in Analysis |publisher=Dover Publications |series=Dover Books on Mathematics |isbn=978-0-486-42875-8 |url={{Google books|D_XBAgAAQBAJ|plainurl=yes}} }}
*{{Citation | last=Hardy | first=G. H. | author-link=G. H. Hardy | year=1916 | title=Weierstrass's nondifferentiable function | journal=Transactions of the American Mathematical Society | publisher=American Mathematical Society | volume=17 | issue=3 | pages=301–325 | url=http://www.ams.org/journals/tran/1916-017-03/S0002-9947-1916-1501044-1/S0002-9947-1916-1501044-1.pdf | doi=10.2307/1989005| jstor=1989005 }}
*{{Citation | last=Weierstrass | first=Karl | author-link=Karl Weierstrass | date=18 July 1872 | title=Über continuirliche Functionen eines reellen Arguments, die für keinen Werth des letzeren einen bestimmten Differentialquotienten besitzen | publisher=Königlich Preußische Akademie der Wissenschaften }}
**{{Citation | last=Weierstrass | first=Karl | year=1895 | title=Mathematische Werke von Karl Weierstrass | chapter=Über continuirliche Functionen eines reellen Arguments, die für keinen Werth des letzeren einen bestimmten Differentialquotienten besitzen | location=Berlin, Germany | publisher=Mayer & Müller | volume=2 | pages=71–74 | chapter-url={{Google books|1FhtAAAAMAAJ|page=71|plainurl=yes}} }}
**English translation: {{Citation | last=Edgar | first=Gerald A. | year=1993 | title=Classics on Fractals | chapter=On continuous functions of a real argument that do not possess a well-defined derivative for any value of their argument | publisher=Addison-Wesley Publishing Company | series=Studies in Nonlinearity | pages=3–9 | isbn=978-0-201-58701-2 }}

== External links ==
* {{MathWorld|id=WeierstrassFunction|title=Weierstrass function}} (a different Weierstrass Function which is also continuous and nowhere differentiable)
* [http://www.apronus.com/math/nodiffable.htm Nowhere differentiable continuous function] proof of existence using [[Banach's contraction principle]].
* [http://www.apronus.com/math/nomonotonic.htm Nowhere monotonic continuous function] proof of existence using the [[Baire category theorem]].
* {{cite web | author = Johan Thim | url = http://epubl.ltu.se/1402-1617/2003/320/index-en.html | title = Continuous Nowhere Differentiable Functions | work = Master Thesis Lulea Univ of Technology 2003 | access-date = 28 July 2006 | archive-date = 22 February 2017 | archive-url = https://web.archive.org/web/20170222141554/http://epubl.ltu.se/1402-1617/2003/320/index-en.html | url-status = dead }}
* [http://jonas.matuzas.googlepages.com/mathematicalbeauties Weierstrass function in the complex plane] {{Webarchive|url=https://web.archive.org/web/20090924022258/http://jonas.matuzas.googlepages.com/mathematicalbeauties |date=24 September 2009 }} Beautiful fractal.
* [https://doi.org/10.1007%2Fs00041-009-9072-2 SpringerLink - Journal of Fourier Analysis and Applications, Volume 16, Number 1] Simple Proofs of Nowhere-Differentiability for Weierstrass's Function and Cases of Slow Growth
* [https://stemblab.github.io/weierstrass/ Weierstrass functions: continuous but not differentiable anywhere]
* [https://web.archive.org/web/20250131212739/https://users.math.msu.edu/users/banelson/teaching/104/104_weierstrass.pdf The Weierstrass Function by Brent Nelson at Berkeley, showing non-differentiable]
{{Fractals}}

[[Category:Theory of continuous functions]]
[[Category:Measure theory]]
[[Category:Real analysis]]
[[Category:Fractal curves]]
[[Category:Types of functions]]