Positive root
= Positive root
{wiki=Root_system#Positive_roots_and_simple_roots}
A choice of positive roots selects exactly one of $\alpha$ and $-\alpha$ and is closed under addition whenever the sum is a root.
= Positive root
{wiki=Root_system#Positive_roots_and_simple_roots}
A choice of positive roots selects exactly one of $\alpha$ and $-\alpha$ and is closed under addition whenever the sum is a root.