# Arithmetic progression

*https://en.wikipedia.org/wiki/Arithmetic_progression*

An **arithmetic progression** or **arithmetic sequence** is a
sequence of
numbers such that the difference between the consecutive terms is constant. For instance, the sequence 5, 7, 9, 11, 13, 15, . . . is an arithmetic progression with a common difference of 2.

If the initial term of an arithmetic progression is and the common difference of successive members is , then the -th term of the sequence () is given by:

- ,

and in general

- .

A finite portion of an arithmetic progression is called a **finite arithmetic progression** and sometimes just called an arithmetic progression. The
sum of a finite arithmetic progression is called an **arithmetic series**.

## Sum

2 | + | 5 | + | 8 | + | 11 | + | 14 | = | 40 |

14 | + | 11 | + | 8 | + | 5 | + | 2 | = | 40 |

16 | + | 16 | + | 16 | + | 16 | + | 16 | = | 80 |

The
sum of the members of a finite arithmetic progression is called an **arithmetic series**. For example, consider the sum:

This sum can be found quickly by taking the number *n* of terms being added (here 5), multiplying by the sum of the first and last number in the progression (here 2 + 14 = 16), and dividing by 2:

In the case above, this gives the equation:

This formula works for any real numbers and . For example:

### Derivation

To derive the above formula, begin by expressing the arithmetic series in two different ways:

Adding both sides of the two equations, all terms involving *d* cancel:

Dividing both sides by 2 produces a common form of the equation:

An alternate form results from re-inserting the substitution: :

Furthermore, the mean value of the series can be calculated via: :

The formula is very similar to the mean of a discrete uniform distribution.

## Product

The
product of the members of a finite arithmetic progression with an initial element *a*_{1}, common differences *d*, and *n* elements in total is determined in a closed expression

where denotes the Gamma function. The formula is not valid when is negative or zero.

This is a generalization from the fact that the product of the progression is given by the factorial and that the product

for positive integers and is given by

### Derivation

where denotes the rising factorial.

By the recurrence formula , valid for a complex number ,

- ,

- ,

so that

for a positive integer and a positive complex number.

Thus, if ,

- ,

and, finally,

### Examples

- Example 1

Taking the example , the product of the terms of the arithmetic progression given by up to the 50th term is

- Example 2

The product of the first 10 odd numbers is given by

- = 654,729,075

## Standard deviation

The standard deviation of any arithmetic progression can be calculated as

where is the number of terms in the progression and is the common difference between terms. The formula is very similar to the standard deviation of a discrete uniform distribution.

## Intersections

The
intersection of any two doubly infinite arithmetic progressions is either empty or another arithmetic progression, which can be found using the
Chinese remainder theorem. If each pair of progressions in a family of doubly infinite arithmetic progressions have a non-empty intersection, then there exists a number common to all of them; that is, infinite arithmetic progressions form a
Helly family.^{
[1]} However, the intersection of infinitely many infinite arithmetic progressions might be a single number rather than itself being an infinite progression.

## History

According to an anecdote of uncertain reliability,^{
[2]} young
Carl Friedrich Gauss in primary school reinvented this method to compute the sum of the integers from 1 through 100, by multiplying *n*/2 pairs of numbers in the sum by the values of each pair *n* + 1.^{[
clarification needed]} However, regardless of the truth of this story, Gauss was not the first to discover this formula, and some find it likely that its origin goes back to the
Pythagoreans in the 5th century BC.^{
[3]} Similar rules were known in antiquity to
Archimedes,
Hypsicles and
Diophantus;^{
[4]} in China to
Zhang Qiujian; in India to
Aryabhata,
Brahmagupta and
Bhaskara II;^{
[5]} and in medieval Europe to
Alcuin,^{
[6]}
Dicuil,^{
[7]}
Fibonacci,^{
[8]}
Sacrobosco^{
[9]}
and to anonymous commentators of
Talmud known as
Tosafists.^{
[10]}

## See also

- Geometric progression
- Harmonic progression
- Triangular number
- Arithmetico-geometric sequence
- Inequality of arithmetic and geometric means
- Primes in arithmetic progression
- Linear difference equation
- Generalized arithmetic progression, a set of integers constructed as an arithmetic progression is, but allowing several possible differences
- Heronian triangles with sides in arithmetic progression
- Problems involving arithmetic progressions
- Utonality

## References

**^**Duchet, Pierre (1995), "Hypergraphs", in Graham, R. L.; Grötschel, M.; Lovász, L. (eds.),*Handbook of combinatorics, Vol. 1, 2*, Amsterdam: Elsevier, pp. 381–432, MR 1373663. See in particular Section 2.5, "Helly Property", pp. 393–394.**^**Hayes, Brian (2006). "Gauss's Day of Reckoning".*American Scientist*.**94**(3): 200. doi: 10.1511/2006.59.200. Archived from the original on 12 January 2012. Retrieved 16 October 2020.**^**Høyrup, J. The “Unknown Heritage”: trace of a forgotten locus of mathematical sophistication. Arch. Hist. Exact Sci. 62, 613–654 (2008). https://doi.org/10.1007/s00407-008-0025-y**^**Tropfke, Johannes (1924).*Analysis, analytische Geometrie*. Walter de Gruyter. pp. 3–15. ISBN 978-3-11-108062-8.**^**Tropfke, Johannes (1979).*Arithmetik und Algebra*. Walter de Gruyter. pp. 344–354. ISBN 978-3-11-004893-3.**^**Problems to Sharpen the Young, John Hadley and David Singmaster,*The Mathematical Gazette*,**76**, #475 (March 1992), pp. 102–126.**^**Ross, H.E. & Knott,B.I (2019) Dicuil (9th century) on triangular and square numbers,*British Journal for the History of Mathematics*, 34:2, 79-94, https://doi.org/10.1080/26375451.2019.1598687**^**Sigler, Laurence E. (trans.) (2002).*Fibonacci's Liber Abaci*. Springer-Verlag. pp. 259–260. ISBN 0-387-95419-8.**^**Katz, Victor J. (edit.) (2016).*Sourcebook in the Mathematics of Medieval Europe and North Africa*. Princeton University Press. pp. 91, 257. ISBN 9780691156859.**^**Stern, M. (1990). 74.23 A Mediaeval Derivation of the Sum of an Arithmetic Progression. The Mathematical Gazette, 74(468), 157-159. doi:10.2307/3619368