2 April 2017

Four

Four color theorem by Kenneth Appel proves that a two-dimensional map, with some limitations, may be colored with four colors without any of the adjacent "countries" be painted with the same color.

A square with the side $4$ has its perimeter equal to its area (without considering the units), ie, $16$.

Any whole number in the form $n^4+4$ is not prime, except for the case $n=1$, where $n^4+4=\left(n^2-2n+2 \right) \left(n^2+2n+2 \right)$. In fact,  is the product of two composite numbers and it is also a composite number.

Any prime number in the form $4k+1$ is the sum of two perfect squares. For example: $13=4 \cdot 3+1=2^2+3^2$.

Multiplying $21,978$ by $4$, it reverses the order of its digits: $21,978 \cdot 4= 87,912$.