# Complex value of z for real and imaginary part and modulus

how do we go about if the function is $g(z) = e^{-2z}$ ?

what values of $z$ makes $g(z)$ real, imaginary, or modulus <1?

$g(z) = e^{-2(x+iy)}$ = $e^{-2x-i2y}$

= $e^{-2x}e^{i(-2y)}$ =$e^{-2x} [\cos(-2x)+i\sin(-2x)]$

Hence, real if $\cos(-2x)=0$ ?

What about $z$ = ? =

• What do you mean by "$z=?=$" – Samrat Mukhopadhyay Feb 11 '15 at 4:54
• the question explicitly asks for values of $z$ where $g$ is real, imaginary, or modulus <1 – rebc Feb 11 '15 at 5:11
• Ok, then you should make it clearer in the question. – Samrat Mukhopadhyay Feb 11 '15 at 5:23

$g(z)=e^{-2x}(\cos 2y-i\sin 2y)$. So $g(z)$ is real iff $e^{-2x}\sin 2y=0\implies y=\frac{n\pi}{2},n\in \mathbb{Z}$ which is a set of horizontal lines in complex plane. Similarly $g(z)$ is imaginary if $y=\frac{2n+1}{4}\pi,\ n\in \mathbb{Z}$. $|g(z)|=e^{-2x}\le 1\ \forall x\in \mathbb{R}^+\cup\{0\}$, i.e. $g(z)$ is inside the unit circle in complex plane $\forall z\in \mathbb{C}$.
• Hi Samrat thanks for the help. however, how do we explicitly find the values of $z$ and not only $y$? – rebc Feb 12 '15 at 2:18
• For the case, say $y=\frac{n\pi}{2}$, the $x$ value can be anything, i.e. $x\in \mathbb{R}$, so $z$ can be any one of the points on the line $y=\frac{n\pi}{2}$ – Samrat Mukhopadhyay Feb 12 '15 at 6:31
• isnt $g(z) = e^{-2x}(cos(-2y)-isin(-2y)$) ? – rebc Feb 12 '15 at 20:50
• @rebc $e^{-i2y}=\cos (-2y)+i\sin(-2y)=\cos(2y)-i\sin(2y)$ – Samrat Mukhopadhyay Feb 13 '15 at 8:54