A set is a collection of objects and a set of numbers is a collection of numbers: $ n \in \\\{ 1, 3, 5, 7 \\\} $ It is conventional to use certain letters to represent infinite sets of numbers. The following expressions involving sets of numbers are all true: - natural numbers: $0 \notin \mathbb{N}$ - natural numbers including zero: $0 \in \mathbb{N_0}$ - integer numbers: $-1 \in \mathbb{Z}$ - rational numbers: $\frac{1}{3} \in \mathbb{Q}$ - real numbers: $\pi \in \mathbb{R}$ - complex complex: $1+j \in \mathbb{C}$ The real numbers ($\mathbb{R}$) can be represented in the form of intervals: - closed range: $-1 \in [-1,1]$ - open range: $-1 \notin ]-1.1]$