##<b>Interval notation.</b> Let $a, b$ be real numbers. #A set $I$ of real numbers is an interval iff for all $x,y\in I$ all real numbers between $x$ and $y$ are also in $I$. #The open interval $(a,b)=$ $\{x\in\m R\ |\ a<x<b\}$ #The closed interval $[a,b]=$ $\{x\in\m R\ |\ a\le x\le b\}$ #The half-open interval $(a,b]=$ $\{x\in\m R\ |\ a< x\le b\}$ #The half-open interval $[a,b)=$ $\{x\in\m R\ |\ a\le x< b\}$ #$(a,\infty)=$ $\{x\in\m R\ |\ a< x\}$ #$(-\infty,b]=$ $\{x\in\m R\ |\ x\le b\}$ #$(-\infty,\infty)=$ $\m R$ (the real line)