# Thread: () {} [] what do they mean again?

1. ## () {} [] what do they mean again?

() this means it includes the #
[] is it the same as {} this? and means it dose not include the # when writing in interval notation

infinity always gets a parentheses correct?

domain look left to right?
range up down?

2. Originally Posted by rj2001
() this means it includes the #
[] is it the same as {} this? and means it dose not include the # when writing in interval notation
no, ( or ) means it does NOT include the number: $\displaystyle x \in (0,1) \implies 0 < x < 1$

[ or ] means it the number is included: $\displaystyle x\in (0, 1] \implies 0 < x \le 1$

{} as i usually see it deals with specific elements.

example, we might write $\displaystyle (- \infty,2) \cup (2, \infty)$ as $\displaystyle \mathbb{R}\backslash \{ 2 \} \mbox{ or } (- \infty, \infty) - \{ 2 \}$ or something like that

infinity always gets a parentheses correct?
yes, always $\displaystyle ( - \infty$ or $\displaystyle \infty)$

domain look left to right?
range up down?
...um, yeah. in a sense...