Trigonometric series explained

In mathematics, a trigonometric series is an infinite series of the form

A _{0} + \sum ^{inf ty} _{n =1} A _{n} \cos{nx} + B _{n} \sin{nx},

where

x is the variable and \{A _{n\}} and \{B _{n\}} are coefficients. It is an infinite version of a trigonometric polynomial.

A trigonometric series is called the Fourier series of the integrable function $f$ if the coefficients have the form:

A ^{2 \pi} _{0} f(x) \cos{nx} dx B _{n = (1)/(\pi)} \displaystyle\int ^{2 \pi} _{0} f(x) \sin{nx} dx

Examples

Every Fourier series gives an example of a trigonometric series.Let the function

f(x) = x on [ - \pi,\pi] be extended periodically (see sawtooth wave). Then its Fourier coefficients are:\begin{align} A _{n} & = (1 \pi\int)/(_{-\pi}) ^{\pi} x \cos{nx}dx = 0 , n \ge 0 . \\[ 4 pt] B _{n} & = (1 \pi\int)/(_{-\pi}) ^{\pi} x \sin{nx} dx \\[ 4 pt] & =- (x)/(n\pi) \cos{nx} + (1 {n)/(^{2\pi}\sin{nx}}) \ vert ^{\pi} _{x =- \pi} \\[ 5 mu] & = (2 ( -1 ) ^{n+1})/(n) , n \ge 1 .\end{align} Which gives an example of a trigonometric series:2 \sum ^{inf ty} _{n =1} (( -1 ) ^{n+1})/(n) \sin{nx} = 2 \sin{x} - (22 \sin{ 2 x})/(+) (23 \sin{ 3 x})/(-) (24 \sin{ 4 x})/(+) …

The converse is false however, not every trigonometric series is a Fourier series. The series

\sum ^{inf ty} _{n =2} (\sin{nx)/() } = \frac + \frac + \frac+\cdotsis a trigonometric series which converges for all x but is not a Fourier series.^[1] Here B _{n = (1)/(log (n))} for n\geq 2 and all other coefficients are zero.

Uniqueness of Trigonometric series

The uniqueness and the zeros of trigonometric series was an active area of research in 19th century Europe. First, Georg Cantor proved that if a trigonometric series is convergent to a function

f(x) on the interval [ 0 , 2 \pi] , which is identically zero, or more generally, is nonzero on at most finitely many points, then the coefficients of the series are all zero.^[2]

Later Cantor proved that even if the set S on which

f is nonzero is infinite, but the derived set S of S is finite, then the coefficients are all zero. In fact, he proved a more general result. Let S0 = S and let Sk+1 be the derived set of Sk. If there is a finite number n for which Sn is finite, then all the coefficients are zero. Later, Lebesgue proved that if there is a countably infinite ordinal α such that Sα is finite, then the coefficients of the series are all zero. Cantor's work on the uniqueness problem famously led him to invent transfinite ordinal numbers, which appeared as the subscripts α in Sα .^[3]

References

• Book: Bari, Nina Karlovna . Nina Bari . 1964 . A Treatise on Trigonometric Series . 1 . Pergamon . Mullins . Margaret F. . limited .
• Book: Zygmund, Antoni . Antoni Zygmund . Trigonometric Series . 1 and 2 . Cambridge University Press . 2nd, reprinted . 0236587 . 1968 . limited .

See also

• Denjoy–Luzin theorem

Notes and References

1. Book: Hardy . Godfrey Harold . G.H. Hardy . Rogosinski . Werner Wolfgang . Werner Wolfgang Rogosinski . 1956 . 1st ed. 1944 . Fourier Series . 3rd . Cambridge University Press . 4–5 . limited .
2. Web site: Set theory and uniqueness for trigonometric series. Alexander S.. Kechris. Caltech. 1997.
3. Cooke. Roger. Uniqueness of trigonometric series and descriptive set theory, 1870–1985. Archive for History of Exact Sciences. 45. 281–334. 1993. 10.1007/BF01886630. 4. 122744778 . ..