2 History of the theory of infinite series

2.1 Convergence criteria

The investigation of the validity of infinite series is considered to begin with GaussJohann Carl Friedrich Gauss (Gauss ( April 30, 1777 February 23, 1855) was a legendary German mathematician, astronomer and physicist with a very wide range of contributions; he is considered to be one of the greatest mathematicians of all time. His name. Euler had already considered the hypergeometric seriesIn mathematics, a hypergeometric series could in principle be any formal power series in which the ratio of successive coefficients a ''a is a rational function of n''. In the case of geometric series the ratio is constant. The series for the exponential

on which Gauss published a memoir in 1812. It established simpler criteria of convergence, and the questions of remainders and the range of convergence.

Cauchy (1821) insisted on strict tests of convergence; he showed that if two series are convergent their product is not necessarily so, and with him begins the discovery of effective criteria. The terms convergence and divergence had been introduced long before by Gregory (1668). Euler and Gauss had given various criteria, and Maclaurin had anticipated some of Cauchy's discoveries. Cauchy advanced the theory of power series by his expansion of a complex function in such a form.

Abel (1826) in his memoir on the series

corrected certain of Cauchy's conclusions, and gave a completely scientific summation of the series for complex values of and . He showed the necessity of considering the subject of continuity in questions of convergence.

Cauchy's methods led to special rather than general criteria, and the same may be said of Raabe (1832), who made the first elaborate investigation of the subject, of De Morgan (from 1842), whose logarithmic test DuBois-Reymond (1873) and Pringsheim (1889) have shown to fail within a certain region; of Bertrand (1842), Bonnet (1843), Malmsten (1846, 1847, the latter without integration);

Stokes (1847), Paucker (1852), Tchebichef (1852), and Arndt

(1853).

General criteria began with Kummer (1835), and have been studied by Eisenstein (1847), Weierstrass in his various contributions to the theory of functions, Dini (1867), DuBois-Reymond (1873), and many others. Pringsheim's (from 1889) memoirs present the most complete general theory.

2.2 Uniform convergence

The theory of uniform convergence was treated by Cauchy (1821), his limitations being pointed out by Abel, but the first to attack it successfully were Stokes and Seidel (1847-48). Cauchy took up the problem again (1853), acknowledging Abel's criticism, and reaching the same conclusions which Stokes had already found. Thomé used the doctrine (1866), but there was great delay in recognizing the importance of distinguishing between uniform and non-uniform convergence, in spite of the demands of the theory of functions.

2.3 Semi-convergence

Semi-convergent series were studied by Poisson (1823), who also gave a general form for the remainder of the Maclaurin formula. The most important solution of the problem is due, however, to Jacobi (1834), who attacked the question of the remainder from a different standpoint and reached a different formula. This expression was also worked out, and another one given, by Malmsten (1847). Schlömilch (Zeitschrift, Vol.I, p. 192, 1856) also improved Jacobi's remainder, and showed the relation between the remainder and Bernoulli's function . Genocchi (1852) has further contributed to the theory.

Among the early writers was Wronski, whose "loi suprême" (1815) was hardly recognized until Cayley (1873) brought it into prominence.

2.4 Interpolation

Interpolation formulas have been given by various writers from Newton to the present time. Lagrange's theorem is well known, although Euler had already given an analogous form, as are also Olivier's formula (1827), and those of Minding (1830), Cauchy (1837), Jacobi (1845), Grunert (1850, 1853), Christoffel (1858), and Mehler (1864).

2.5 Fourier series

Fourier series were being investigated

as the result of physical considerations at the same time that Gauss, Abel, and Cauchy were working out the theory of infinite series. Series for the expansion of sines and cosines, of multiple arcs in powers of the sine and cosine of the arc had been treated by

Jakob Bernoulli (1702) and his brother Johann Bernoulli (1701) and still

earlier by Viète. Euler and Lagrange simplified the subject, as did Poinsot, Schröter, Glaisher, and Kummer.

Fourier (1807) set for himself a different problem, to expand a given function of in terms of the sines or cosines of multiples of , a problem which he embodied in his Théorie analytique de la Chaleur (1822). Euler had already given the formulas for determining the coefficients in the series; Fourier was the first to assert and attempt to prove the general theorem. Poisson (1820-23) also attacked the problem from a different standpoint. Fourier did not, however, settle the question of convergence of his series, a matter left for Cauchy (1826) to attempt and for Dirichlet (1829) to handle in a thoroughly scientific manner (see convergence of Fourier series). Dirichlet's treatment ( Crelle, 1829), of trigonometric series was the subject of criticism and improvement by Riemann (1854), Heine, Lipschitz, Schläfli , and

DuBois-Reymond . Among other prominent contributors to the theory of

trigonometric and Fourier series have were Dini , Hermite, Halphen , Krause, Byerly and Appell .

3 Some types of infinite series

• A geometric series is one where each successive term is produced by multiplying the previous term by a constant number. Example:

• The harmonic series is the series

• An alternating series is a series where terms alternate signs. Example:

4 Convergence criteria

1. Zero test: If the series ∑ a_n converges, then the sequence (a_n) converges to 0 for n→∞; the converse is in general not true.
2. Comparison test: If ∑ b_n is an absolutely convergent series such that |a_n| ≤ b_n for all sufficiently large n, then ∑ a_n converges absolutely as well. If all the b_n are positive, |a_n| ≥ b_n for all sufficiently large n and ∑ b_n diverges, then ∑ a_n also fails to converge absolutely (though it could still be conditionally convergent, if the a_n alternate in sign).
3. Ratio test: If there exists a constant C < 1 such that |a_n+1| ≤ C |a_n| for all sufficiently large n, then ∑ a_n converges absolutely.
4. Root test: If there exists a constant C < 1 such that |a_n|^1/n ≤ C for all sufficiently large n, then ∑ a_n converges absolutely.
5. Integral test: if f(x) is a positive monotone decreasing function defined on the interval [1, ∞[ with f(n) = a_n for all n, then ∑ a_n converges if and only if the integral ∫₁^∞ f(x) dx is finite.
6. Alternating series test: A series of the form ∑ (-1)ⁿ a_n (with a_n ≥ 0) is called alternating. Such a series converges if the sequence a_n is monotone decreasing and converges to 0. The converse is in general not true.
7. For some specific types of series there are more specialized convergence tests, for instance for Fourier series there is the Dini test.

5 Examples

The series

converges if r > 1 and diverges for r ≤ 1, which can be shown with the integral criterion 5) from above. As a function of r, the sum of this series is Riemann's zeta function.

The geometric series

converges if and only if |z| < 1.

The telescoping series

converges if the sequence b_n converges to a limit L as n goes to infinity. The value of the series is then b₁ − L.

The power series of

converges to .

6 Absolute convergence

The sum

is said to converge absolutely if the series of absolute values

converges. In this case, the original series, and all reorderings of it, converge, and converge towards the same sum.

If a series converges, but not absolutely, then one can always find a reordering of the terms so that the reordered series diverges. Even more: if the a_n are real and S is any real number, one can find a reordering so that the reordered series converges with limit S (Riemann).

7 Power series

Several important functions can be represented as Taylor series; these are infinite series involving powers of the independent variable and are also called power series. See also radius of convergence.

Historically, mathematicians such as Leonhard Euler operated liberally with infinite series, even if they were not convergent. When calculus was put on a sound and correct foundation in the nineteenth century, rigorous proofs of the convergence of series were always required. However, the formal operation with non-convergent series has been retained in rings of formal power series which are studied in abstract algebra. Formal power series are also used in combinatorics to describe and study sequences that are otherwise difficult to handle; this is the method of generating functions.

8 Generalizations

The notion of series can be defined in every abelian topological group; the most commonly encountered case is that of series in a Banach space.

There is no serious definition for an infinite sum over an uncountable set. For example if X is a set and f a function on X taking non-negative real values, such that

for any countable subset Y of X, with A an absolute constant, it follows that f(x) = 0 for all x outside some countable subset of X. In other words, infinite sums of uncountably many non-negative reals make sense only in the case that this is a conventional convergent infinite series, extended by the value 0 to an uncountable set.

Asymptotic series, otherwise asymptotic expansions, are not typically convergent infinite series, but sequences of finite approximations each of which is a good asymptotic representation.

9 See also

divergent series

Calculus *

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