# Definition:Ordinal Sum

Let $y$ be an ordinal.
Let $\left\langle{A_x}\right\rangle$ be a sequence of ordinals.
The ordinal sum of $A_x$ is denoted $\displaystyle \sum_{x \mathop = 1}^y A_x$ and defined using Transfinite Recursion on $y$ as follows:
$\displaystyle \sum_{x \mathop = 1}^\varnothing A_x = \varnothing$
$\displaystyle \sum_{x \mathop = 1}^{z^+} A_x = \sum_{x \mathop = 1}^z \left({A_x}\right) + A_{z^+}$
$\displaystyle \sum_{x \mathop = 1}^y A_x = \bigcup_{z \mathop \in y} \left({\sum_{x \mathop = 1}^z A_x}\right)$ for limit ordinals $y$.