Definition:Rising Factorial

Definition
Let $x$ be a real number (but usually an integer).

Let $k$ be a positive integer.

Then $x$ to the (power of) $k$ rising is:
 * $\displaystyle x^{\overline k} := \prod_{j \mathop = 0}^{k-1} \left({x + j}\right) = x \left({x + 1}\right) \cdots \left({x + k - 1}\right)$

For other values of $k$, this formula may be used:
 * $\displaystyle x^{\overline k} = \frac {\Gamma \left({x + k}\right)}{\Gamma \left({x}\right)}$

where $\Gamma$ signifies the Gamma function.

It is clear from the definition of the factorial that:
 * $1^{\overline k} = k!$
 * $k^{\overline 1} = k$

Also:
 * $x^{\overline k} = \left({x + k - 1}\right)^{\underline k}$

where $x^{\underline k}$ is the $k$th falling factorial power of $x$.

Also known as
This is referred to as the $k$th rising factorial power of $x$.

It can also be referred to as the $k$th rising factorial of $x$.

Also see

 * Definition:Falling Factorial
 * Definition:Factorial
 * Definition:Gamma Function