# Math Help - Help please

Discuss whether or not the given function is injective, surjective, and/or bijective.

f: N into Q, where f(n) = 1/n

g: X subset of R^3 into [1,infinity), where g(x,y,z) = e^sqaure root of x^2+y^2+z^2-4

h: X subset of R^3 into R^3, where h(x) = 2x/2 norm of x

2. Originally Posted by jburks100
Discuss whether or not the given function is injective, surjective, and/or bijective.

$f: \mathbb{N}\longrightarrow\mathbb{Q}$, where $f(n) = 1/n$

$g: X\subseteq\mathbb{R}^3 \longrightarrow [1,\infty)$, where $g(x,y,z) = e^{\sqrt{x^2+y^2+z^2-4}}$

$h: X\subseteq\mathbb{R}^3\longrightarrow\mathbb{R}^3$, where $h(x) = \frac{2x}{||x||_2}$
$f$ is certainly injective, because it has a well-defined inverse. It is not surjective because not every point in $\mathbb{Q}$ is hit. It's not bijective because both aren't satisfied.

$g$ is not injective because the points $(2,3,4)$ and $(2,4,3)$ (for example) yield the same value of $g$. It is, however, surjective, as every point in $[1,\infty)$ is hit. It's not bijective because both aren't satisfied.

$h$ isn't injective, because, for example, $h(1,2,3)$ and $h(2,4,6)$ both yield the same value. It isn't surjective either, because the only points in $\mathbb{R}^3$ that are hit are vectors of length $2$, which certainly don't compose all of 3-space.

PS: Make sure that the way I interpreted your functions is correct.