Power series

Examples

Polynomial

Every polynomial of degree d can be expressed as a power series around any center c, where all terms of degree higher than d have a coefficient of zero. For instance, the polynomial f(x) = x^2 + 2x + 3 can be written as a power series around the center c = 0 as f(x) = 3 + 2 x + 1 x^2 + 0 x^3 + 0 x^4 + \cdots or around the center c = 1 as f(x) = 6 + 4(x - 1) + 1(x - 1)^2 + 0(x - 1)^3 + 0(x - 1)^4 + \cdots.

One can view power series as being like "polynomials of infinite degree", although power series are not polynomials in the strict sense.

Geometric series, exponential function and sine

The geometric series formula \frac{1}{1 - x} = \sum_{n=0}^\infty x^n = 1 + x + x^2 + x^3 + \cdots, which is valid for |x| , is one of the most important examples of a power series, as are the exponential function formula e^x = \sum_{n=0}^\infty \frac{x^n}{n!} = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots and the sine formula \sin(x) = \sum_{n=0}^\infty \frac{(-1)^n x^{2n+1}}{(2n + 1)!} = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots, valid for all real x. These power series are examples of Taylor series (or, more specifically, of Maclaurin series).

On the set of exponents

Negative powers are not permitted in an ordinary power series; for instance, x^{-1} + 1 + x^{1} + x^{2} + \cdots is not considered a power series (although it is a Laurent series). Similarly, fractional powers such as x^\frac{1}{2} are not permitted; fractional powers arise in Puiseux series. The coefficients a_n must not depend on x, thus for instance \sin(x) x + \sin(2x) x^2 + \sin(3x) x^3 + \cdots is not a power series.

Radius of convergence

A power series \sum_{n=0}^\infty a_n(x-c)^n is convergent for some values of x, which always include , since, for this value of x, the series reduces to its first term .

The series may diverge for other values of x, possibly all of them. If c is not the only value of convergence, then there is always a number r with 0 r. The number r is called the radius of convergence of the power series; in general it is given as r = \liminf_{n\to\infty} \left|a_n\right|^{-\frac{1}{n}} or, equivalently, r^{-1} = \limsup_{n\to\infty} \left|a_n\right|^\frac{1}{n}. This is the Cauchy–Hadamard theorem; see limit superior and limit inferior for an explanation of the notation. The relation r^{-1} = \lim_{n\to\infty}\left|{a_{n+1}\over a_n}\right| is also satisfied, if this limit exists.

The set of the complex numbers such that {{math|

For , there is no general statement on the convergence of the series. However, Abel's theorem states that if the series is convergent for some value z such that , then the sum of the series for is the limit of the sum of the series for where t is a real variable less than that tends to .

Operations on power series

Addition and subtraction

When two functions f and g are decomposed into power series around the same center c, the power series of the sum or difference of the functions can be obtained by termwise addition and subtraction. That is, if f(x) = \sum_{n=0}^\infty a_n (x - c)^n and g(x) = \sum_{n=0}^\infty b_n (x - c)^n then f(x) \pm g(x) = \sum_{n=0}^\infty (a_n \pm b_n) (x - c)^n.

The sum of two power series will have a radius of convergence of at least the smaller of the two radii of convergence of the two series, but possibly larger than either of the two. For instance it is not true that if two power series \sum_{n=0}^\infty a_n x^n and \sum_{n=0}^\infty b_n x^n have the same radius of convergence, then \sum_{n=0}^\infty \left(a_n + b_n\right) x^n also has this radius of convergence: if a_n = (-1)^n and b_n = (-1)^{n+1} \left(1 - \frac{1}{3^n}\right), for instance, then both series have the same radius of convergence of 1, but the series \sum_{n=0}^\infty \left(a_n + b_n\right) x^n = \sum_{n=0}^\infty \frac{(-1)^n}{3^n} x^n has a radius of convergence of 3.

Multiplication and division

With the same definitions for f(x) and g(x), the power series of the product and quotient of the functions can be obtained as follows: \begin{align} f(x)g(x) &= \biggl(\sum_{n=0}^\infty a_n (x-c)^n\biggr)\biggl(\sum_{n=0}^\infty b_n (x - c)^n\biggr) \ &= \sum_{i=0}^\infty \sum_{j=0}^\infty a_i b_j (x - c)^{i+j} \ &= \sum_{n=0}^\infty \biggl(\sum_{i=0}^n a_i b_{n-i}\biggr) (x - c)^n. \end{align}

The sequence m_n = \sum_{i=0}^n a_i b_{n-i} is known as the Cauchy product of the sequences a_n and b_n.

For division, if one defines the sequence d_n by \frac{f(x)}{g(x)} = \frac{\sum_{n=0}^\infty a_n (x - c)^n}{\sum_{n=0}^\infty b_n (x - c)^n} = \sum_{n=0}^\infty d_n (x - c)^n then f(x) = \biggl(\sum_{n=0}^\infty b_n (x - c)^n\biggr)\biggl(\sum_{n=0}^\infty d_n (x - c)^n\biggr) and one can solve recursively for the terms d_n by comparing coefficients.

Solving the corresponding equations yields the formulae based on determinants of certain matrices of the coefficients of f(x) and g(x) d_0=\frac{a_0}{b_0} d_n=\frac{1}{b_0^{n+1}} \begin{vmatrix} a_n &b_1 &b_2 &\cdots&b_n \ a_{n-1}&b_0 &b_1 &\cdots&b_{n-1}\ a_{n-2}&0 &b_0 &\cdots&b_{n-2}\ \vdots &\vdots&\vdots&\ddots&\vdots \ a_0 &0 &0 &\cdots&b_0\end{vmatrix}

Differentiation and integration

Once a function f(x) is given as a power series as above, it is differentiable on the interior of the domain of convergence. It can be differentiated and integrated by treating every term separately since both differentiation and integration are linear transformations of functions: \begin{align} f'(x) &= \sum_{n=1}^\infty a_n n (x - c)^{n-1} = \sum_{n=0}^\infty a_{n+1} (n + 1) (x - c)^n, \ \int f(x),dx &= \sum_{n=0}^\infty \frac{a_n (x - c)^{n+1}}{n + 1} + k = \sum_{n=1}^\infty \frac{a_{n-1} (x - c)^n}{n} + k. \end{align}

Both of these series have the same radius of convergence as the original series.

Analytic functions

Main article: Analytic function

A function f defined on some open subset U of R or C is called analytic if it is locally given by a convergent power series. This means that every a ∈ U has an open neighborhood V ⊆ U, such that there exists a power series with center a that converges to f(x) for every x ∈ V.

Every power series with a positive radius of convergence is analytic on the interior of its region of convergence. All holomorphic functions are complex-analytic. Sums and products of analytic functions are analytic, as are quotients as long as the denominator is non-zero.

If a function is analytic, then it is infinitely differentiable, but in the real case the converse is not generally true. For an analytic function, the coefficients a**n can be computed as a_n = \frac{f^{\left( n \right)} \left( c \right)}{n!}

where f^{(n)}(c) denotes the nth derivative of f at c, and f^{(0)}(c) = f(c). This means that every analytic function is locally represented by its Taylor series.

The global form of an analytic function is completely determined by its local behavior in the following sense: if f and g are two analytic functions defined on the same connected open set U, and if there exists an element c ∈ U such that for all n ≥ 0, then for all x ∈ U.

If a power series with radius of convergence r is given, one can consider analytic continuations of the series, that is, analytic functions f which are defined on larger sets than {{math|{{mset| x |

The power series expansion of the inverse function of an analytic function can be determined using the Lagrange inversion theorem.

Behavior near the boundary

The sum of a power series with a positive radius of convergence is an analytic function at every point in the interior of the disc of convergence. However, different behavior can occur at points on the boundary of that disc. For example:

1. Divergence while the sum extends to an analytic function: \sum_{n=0}^{\infty}z^n has radius of convergence equal to 1 and diverges at every point of |z|=1. Nevertheless, the sum in |z| is \frac{1}{1-z}, which is analytic at every point of the plane except for z=1.
2. Convergent at some points divergent at others: \sum_{n=1}^{\infty}\frac{z^n}{n} has radius of convergence 1. It converges for z=-1, while it diverges for z=1.
3. Absolute convergence at every point of the boundary: \sum_{n=1}^{\infty}\frac{z^n}{n^2} has radius of convergence 1, while it converges absolutely, and uniformly, at every point of |z|=1 due to Weierstrass M-test applied with the hyper-harmonic convergent series \sum_{n=1}^{\infty}\frac{1}{n^2}.
4. Convergent on the closure of the disc of convergence but not continuous sum: Sierpiński gave an example of a power series with radius of convergence 1, convergent at all points with |z|=1, but the sum is an unbounded function and, in particular, discontinuous. A sufficient condition for one-sided continuity at a boundary point is given by Abel's theorem.

Generating series

Main article: Generating function

It is common to associate to any sequence of numbers its generating series \textstyle \sum_{n=0}^\infty a_nx^n or its exponential generating series \textstyle \sum_{n=0}^\infty \frac{a_n}{n!}x^n.

When the radius of convergence is positive, the series defines an analytic function. Generating series and the analytic functions that they define are a powerful tool for studying numerical series and their asymptotic behavior. This is used, in particular, in analytic combinatorics, where the whole power of complex analysis is used for getting accurate estimates of the number of combinatorial structures of a given type as a function of their size.

Formal power series

Main article: Formal power series

In abstract algebra, one attempts to capture the essence of power series without being restricted to the fields of real and complex numbers, and without the need to talk about convergence. This leads to the concept of formal power series, a concept of great utility in algebraic combinatorics.

Power series in several variables

An extension of the theory is necessary for the purposes of multivariable calculus. A power series is here defined to be an infinite series of the form f(x_1, \dots, x_n) = \sum_{j_1, \dots, j_n = 0}^\infty a_{j_1, \dots, j_n} \prod_{k=1}^n (x_k - c_k)^{j_k}, where is a vector of natural numbers, the coefficients a(j1, …, j**n) are usually real or complex numbers, and the center and argument are usually real or complex vectors. The symbol \Pi is the product symbol, denoting multiplication. In the more convenient multi-index notation this can be written f(x) = \sum_{\alpha \in \N^n} a_\alpha (x - c)^\alpha. where \N is the set of natural numbers, and so \N^n is the set of ordered n-tuples of natural numbers.

The theory of such series is trickier than for single-variable series, with more complicated regions of convergence. For instance, the power series \sum_{n=0}^\infty x_1^n x_2^n is absolutely convergent in the set { (x_1, x_2): |x_1 x_2| between two hyperbolas. (This is an example of a log-convex set, in the sense that the set of points (\log |x_1|, \log |x_2|), where (x_1, x_2) lies in the above region, is a convex set. More generally, one can show that when c=0, the interior of the region of absolute convergence is always a log-convex set in this sense.) On the other hand, in the interior of this region of convergence one may differentiate and integrate under the series sign, just as one may with ordinary power series.

Order of a power series

Let α be a multi-index for a power series f(x1, x2, …, x**n). The order of the power series f is defined to be the least value r such that there is a**α ≠ 0 with r = |\alpha| = \alpha_1 + \alpha_2 + \cdots + \alpha_n, or \infty if f ≡ 0. In particular, for a power series f(x) in a single variable x, the order of f is the smallest power of x with a nonzero coefficient. This definition readily extends to Laurent series.

Notes

References

References

1. Howard Levi. (1967). "Polynomials, Power Series, and Calculus". Van Nostrand.
2. Erwin Kreyszig, Advanced Engineering Mathematics, 8th ed, page 747
3. Wacław Sierpiński. (1916). "Sur une série potentielle qui, étant convergente en tout point de son cercle de convergence, représente sur ce cercle une fonction discontinue. (French)". Palermo Rend..
4. (1948). "Convex functions". Bulletin of the American Mathematical Society.