Concept history

Added exercise "Somme de nombres pairs"

Concept created

An even number is an [[Integer|integer]] of the form $n=2k$ where $k$ is another integer. 

##### Examples
- $2=2\times 1$, $4=2\times 2$, $6=2\times 3$, are even.
- $2026=2\times 2013$ is even.
- $13$ is not even. It is [[Odd number|odd]].