Sum: Addition of a sequence of numbers

This operation is called additive summation or addition. There are many ways of writing sums, including:

• Addition (${\displaystyle 2+4+6=12}$)
• Summation (${\displaystyle \sum _{k=1}^{3}k=1+2+3=6}$)
• Code:

Sum = 0
For I = M to N
Sum = Sum + X(I)
Next I (in Visual BASIC)

Sigma notation is a mathematical notation to write long sums in a short way. Sigma notation uses the Greek letter Sigma (${\displaystyle \Sigma }$ ), and takes upper and lower bounds which tell us where the sum begins and where it ends. The lower bound usually has a variable (called the index, often denoted by ${\displaystyle i}$ , ${\displaystyle j}$  or ${\displaystyle k}$ ) along with a value, such as "${\displaystyle i=2}$ ". This tells us that the summation begins at 2, and goes up by 1 until it reaches the number on the top.

${\displaystyle \sum _{i=1}^{n}0=0}$ 

${\displaystyle \sum _{i=1}^{n}1=n}$ 

${\displaystyle \sum _{i=1}^{n}n=n^{2}}$ 

${\displaystyle \sum _{i=1}^{n}i={\frac {n(n+1)}{2}}}$ 

${\displaystyle \sum _{i=1}^{n}i^{2}={\frac {n(n+1)(2n+1)}{6}}}$ 

${\displaystyle \sum _{i=1}^{n}i^{3}={\frac {n^{2}(n+1)^{2}}{4}}}$ 

${\displaystyle \sum _{i=1}^{\infty }a_{i}=\lim _{t\to \infty }\sum _{i=1}^{t}a_{i}}$ 

Sums are used to represent series and sequences. For example:

${\displaystyle \sum _{i=1}^{4}{\frac {1}{2^{i}}}={\frac {1}{2^{1}}}+{\frac {1}{2^{2}}}+{\frac {1}{2^{3}}}+{\frac {1}{2^{4}}}}$ 

The geometric series of a repeating decimal can be represented in summation. For example:

${\displaystyle \sum _{i=1}^{\infty }{\frac {3}{10^{i}}}=0.333333...={\frac {1}{3}}}$ 

The concept of an integral is a limit of sums, with the area under a curve being defined as:

${\displaystyle \lim _{n\to \infty }\sum _{i=1}^{n}f(x_{i}^{*})\Delta x}$ 

• Product (mathematics)

• Nicholas J. Higham, "The accuracy of floating point summation", SIAM J. Scientific Computing 14 (4), 783–799 (1993).

•   Media related to Summation at Wiki Commons
• Sigma Notation Archived 2015-09-21 at the Wayback Machine on PlanetMath
• Derivation of Polynomials to Express the Sum of Natural Numbers with Exponents Archived 2013-02-18 at the Wayback Machine