In this new version of Reference for Functional Equations I added several more z-transform pairs. I also started to add binomial transform pairs. The definition for the binomial is not consistent among different authors. I arbitrarily chose one, and later I changed it. I will probably change it again. Several typos were fixed. I am working on a system to include proofs so that the tables can be checked more easily.

# Tag: functional equations

## A Reference for Functional Equations

I have not posted in a while; one reason is that I got sucked into some interesting mathematics; the work-in-progress Reference for Functional Equations is the result. If you are interested in such things – have a look.

## Difference and Functional Equations Reference

Original image by openDemocracy.

The document below contains tables and formulas useful for working with functional equations, especially difference equations, and to a lesser extent, quotient equations (where differences are replaced by quotients).

The reference contains tables for forward differences, (indefinite) sums, quotients, and products. There is also a table of z-transforms, binomial transfroms, formulas for converting certain kinds of functional equations to difference equations and some discrete Taylor series. There are more than 500 formulas in its 65 pages.

This is a work in progress, so be sure to read the preface (which highlights some of the issues with this document). If you find any errors, please comment below.

## Changes

### Version 1.1

- Added Exponential Sums to differences and sums.
- Additions to the z-transform table.
- Added Binomial Transform pairs.

### Version 1.2

- Expanded the section on the discrete power functions.
- Expanded the section that explain the sue of constants in the table.
- Added forms involving the following expressions to the sum (x + h) tables:
- Updated all the graphs, and added some new ones.
- Reorganized slightly, and fixed some typos.
- Added a few examples, explanations, and additional notations in the sum () tables.

### Version 1.3

- Made several corrections.
- Added the chain and substitution rules for arithmetic differences.
- Added table of functions for reference.
- Expanded the introduction somewhat.
- Added definition for arithmetic difference analogs.
- Added rules for manipulating arithmetic difference analogs.
- Added several new entries, including several functions whose sums can be expressed as the sum of .
- Expressed the G-function (sum of the Gamma function) as a product of known functions, and replaced its notation. The notation G(x) is now used for the Barnes G-function.

### Version 1.4

- Made, as always, a few corrections.
- Made some minor additions to many of the tables.
- Added the tangent sum function. There are still many details to sort out for this and related functions (cot, sec, csc, their hyperbolic counterparts, , and so on), and hence these sections are still messy. These will be cleaned up as the details become clear.
- Replaced some of the statements on periodic, odd, and even functions with precise versions. The previous ones were only correct up to a periodic function.
- Added the derangement function (expressed in terms of the incomplete gamma function), as well as some related Taylor series.
- Since I included the definitions of analog functions, I discovered that the intuitive notion of analogs did not correspond to the definition. Thus, the analogs of ln x and atan x have been removed / replaced. These might re-appear if the definition of analog functions is suitably adjusted.
- Made many statements on the z-transform more precise.
- Made some notations more consistent with standard notation.

### Version 1.5

- Made a small correction for the binomial law for discrete powers.

## Download

DiscreteCalculusTables_1_5 (PDF 4.6 MB).