In mathematics the indefinite sum operator (also known as the antidifference operator), denoted by or ,[1][2][3] is the linear operator, inverse of the forward difference operator . It relates to the forward difference operator as the indefinite integral relates to the derivative. Thus
More explicitly, if , then
If F(x) is a solution of this functional equation for a given f(x), then so is F(x)+C for any constant C. Therefore each indefinite sum actually represents a family of functions, differing by an additive constant.
Fundamental theorem of discrete calculus
Indefinite sums can be used to calculate definite sums with the formula:[4]
Definitions
Laplace summation formula
- where are the Bernoulli numbers of the second kind.[5]
Newton's formula
- where is the falling factorial.
Faulhaber's formula
provided that the right-hand side of the equation converges.
Mueller's formula
If then
- [6]
Ramanujan's formula
Connection to the Ramanujan summation
Often the constant C in indefinite sum is fixed from the following equation:
or
In this case, where
then Ramanjuan's sum is defined as
or
- [7][8]
Summation by parts
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Indefinite summation by parts:
Definite summation by parts:
Period rule
If is a period of function then
Alternative usage
Some authors use the phrase "indefinite sum" to describe a sum in which the numerical value of the upper limit is not given. e.g.
In this case a closed form expression F(k) for the sum is a solution of
- which is called the telescoping equation.[9] It is inverse to backward difference operator.
It is related to the forward antidifference operator using the fundamental theorem of discrete calculus described earlier.
List of indefinite sums
This is a list of indefinite sums of various functions. Not every function has an indefinite sum that can be expressed in terms of elementary functions.
Antidifferences of rational functions
- where , the generalized to real order Bernoulli polynomials.
- where is the polygamma function.
- where is the digamma function.
Antidifferences of exponential functions
Antidifferences of logarithmic functions
Antidifferences of hyperbolic functions
- where is the q-digamma function.
Antidifferences of trigonometric functions
- where is the q-digamma function.
Antidifferences of inverse hyperbolic functions
Antidifferences of inverse trigonometric functions
Antidifferences of special functions
- where is the incomplete gamma function.
- where is the falling factorial.
- (see super-exponential function)
See also
References
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Further reading
- "Difference Equations: An Introduction with Applications", Walter G. Kelley, Allan C. Peterson, Academic Press, 2001, ISBN 0-12-403330-X
- Markus Müller. How to Add a Non-Integer Number of Terms, and How to Produce Unusual Infinite Summations
- Markus Mueller, Dierk Schleicher. Fractional Sums and Euler-like Identities
- S. P. Polyakov. Indefinite summation of rational functions with additional minimization of the summable part. Programmirovanie, 2008, Vol. 34, No. 2.
- "Finite-Difference Equations And Simulations", Francis B. Hildebrand, Prenctice-Hall, 1968
- ↑ Template:PlanetMath
- ↑ On Computing Closed Forms for Indefinite Summations. Yiu-Kwong Man. J. Symbolic Computation (1993), 16, 355-376
- ↑ "If Y is a function whose first difference is the function y, then Y is called an indefinite sum of y and denoted Δ-1y" Introduction to Difference Equations, Samuel Goldberg
- ↑ "Handbook of discrete and combinatorial mathematics", Kenneth H. Rosen, John G. Michaels, CRC Press, 1999, ISBN 0-8493-0149-1
- ↑ Bernoulli numbers of the second kind on Mathworld
- ↑ Markus Müller. How to Add a Non-Integer Number of Terms, and How to Produce Unusual Infinite Summations (note that he uses a slightly alternative definition of fractional sum in his work, i.e. inverse to backwards difference, hence 1 as the lower limit in his formula)
- ↑ Bruce C. Berndt, Ramanujan's Notebooks, Ramanujan's Theory of Divergent Series, Chapter 6, Springer-Verlag (ed.), (1939), pp. 133–149.
- ↑ Éric Delabaere, Ramanujan's Summation, Algorithms Seminar 2001–2002, F. Chyzak (ed.), INRIA, (2003), pp. 83–88.
- ↑ Algorithms for Nonlinear Higher Order Difference Equations, Manuel Kauers