# Exponentiation

Exponentiation (power) is an arithmetic operation on numbers. It is repeated multiplication, just as multiplication is repeated addition. People write exponentiation with upper index. This looks like this: $x^y$. Sometimes it is not possible. Then people write powers using the ^ sign: 2^3 means $2^3$.

The number $x$ is called base, and the number $y$ is called exponent. For example, in $2^3$, 2 is the base and 3 is the exponent.

To calculate $2^3$ a person must multiply the number 2 by itself 3 times. So $2^3=2 \cdot 2 \cdot 2$. The result is $2 \cdot 2 \cdot 2=8$. The equation could be read out loud in this way: 2 raised to the power of 3 equals 8.

Examples:

• $5^3=5\cdot{} 5\cdot{} 5=125$
• $x^2=x\cdot{} x$
• $1^x = 1$ for every number x

If the exponent is equal to 2, then the power is called square because the area of a square is calculated using $a^2$. So

$x^2$ is the square of $x$

If the exponent is equal to 3, then the power is called cube because the volume of a cube is calculated using $a^3$. So

$x^3$ is the cube of $x$

If the exponent is equal to -1 then the person must calculate the inverse of the base. So

$x^{-1}=\frac{1}{x}$

If the exponent is an integer and is less than 0 then the person must invert the number and calculate the power. For example:

$2^{-3}=\left(\frac{1}{2}\right)^3=\frac{1}{8}$

If the exponent is equal to $\frac{1}{2}$ then the result of exponentiation is the square root of the base. So $x^{\frac{1}{2}}=\sqrt{x}.$ Example:

$4^{\frac{1}{2}}=\sqrt{4}=2$

Similarly, if the exponent is $\frac{1}{n}$ the result is the nth root, so:

$a^{\frac{1}{n}}=\sqrt[n]{a}$

If the exponent is a rational number $\frac{p}{q}$, then the result is the qth root of the base raised to the power of p, so:

$a^{\frac{p}{q}}=\sqrt[q]{a^p}$

The exponent may not even be rational. To raise a base a to an irrational xth power, we use an infinite sequence of rational numbers (xi), whose limit is x:

$x=\lim_{n\to\infty}x_n$

like this:

$a^x=\lim_{n\to\infty}a^{x_n}$

There are some rules which help to calculate powers:

• $\left(a\cdot b\right)^n = a^n\cdot{}b^n$
• $\left(\frac{a}{b}\right)^n = \frac{a^n}{b^n},\quad b\neq 0$
• $a^r \cdot{} a^s = a^{r+s}$
• $\frac{a^r}{a^s} = a^{r-s},\quad a\neq 0$
• $a^{-n} = \frac{1}{a^n},\quad a\neq 0$
• $\left(a^r\right)^s = a^{r\cdot s}$
• $a^0 = 1,\quad a\neq 0$: Where the base is greater than 1 and the exponent is 0, the answer is 1. If the base and exponent are both 0, the answer is undefined.

It is possible to calculate exponentiation of matrices. The matrix must be square. For example: $I^2=I \cdot I=I$.