Does $i^i$ and $i^{1\over e}$ have more than one root in $[0, 2 \pi]$
How to find all roots if power contains imaginary or irrational power of complex number? How do I find all roots of the following complex numbers? $$(1 + i)^i, (1 + i)^e, (1 + i)^{ i\over e}$$
EDIT:: Find the value of $i^i$ and $i^{1\over e}$ in $[0, 4\pi]$?
Since you probably mean the multi-valued complex exponential, you can go straight to the definition:
$$ x^y := \exp(y \log x) $$
The multi-valued complex logarithm can be computed as
$$ \log x = \ln |x| + \mathbf{i} \arg(x) $$
where $\arg(x)$ is the polar angle of $x$. This, of course, takes values in the set
$$ \arg(x) = \mathop{\text{Arg}}(x) + 2 \pi n $$
where $n$ is an integer, and $\mathop{\text{Arg}}$ is the "principal value". For your particular question, all you need to know about the idea of a principal value is that $\mathop{\text{Arg}}(x)$ is a particular choice of polar angle for $x$; here it doesn't matter which one you choose.
To demonstrate that this reproduces what you already know, consider the case of $(-8)^{1/3}$. In this case, we have:
- $\mathop{\text{Arg}}(x) = \pi$
- $\arg(x) = \pi + 2 \pi n$
- $\log(x) = \ln(8) + \mathbf{i} \pi (1 + 2n)$
- $\exp(y \log x) = \exp((1/3) \ln(8) + \mathbf{i} \pi (1 + 2n)/3) = \exp(\ln(2)) \exp(\mathbf{i} \pi (1 + 2n)/3)$
- $x^y = 2 (\cos(\frac{1+2n}{3} \pi) + \mathbf{i} \sin(\frac{1 + 2n}{3} \pi))$
and so the values for $(-8)^{1/3}$ are all of the numbers $2 \exp(\mathbf{i} \pi \frac{1 + 2n}{3})$ where $n$ is an integer. $n=1$ gives you the familiar value $-2$. $n=0$ and $n=2$ give the other two complex cube roots of $-8$. $n=4$ also gives $-2$, because $3 \pi \mathbf{i}$ and $\pi \mathbf{i}$ are the same angle. In fact, as $n$ ranges over all integers, only three distinct values are produced. This agrees with the familiar fact that nonzero complex numbers have three cube roots.
So we want a general rule for evaluating formal expressions $z^w$, where $z=r\, e^{i\phi}\ne 0$ and $w=u+iv$ are complex numbers.
Depending on circumstances one wants the full infinite set of values for $z^w$, which arises from the multivaluedness of the logarithm function in the complex domain, or one wants only the so called principal value of $z^w$. Denote by $\dot{\mathbb C}$ the punctured compex plane and by ${\mathbb C}^-$ the complex plane slit along the negative real axis. Then $\arg:\ \dot{\mathbb C}\to{\mathbb R}/(2\pi{\mathbb Z})$ is the multivalued argument function and ${\rm Arg}:\ {\mathbb C}^-\to\ ]{-\pi},\pi[\ $ the principal value of the argument. Correspondingly one has two forms of the logarithm: $$\log z:=\log|z| + i\arg(z)\ ,\qquad{\rm Log}z:=\log|z| + i{\rm Arg}(z)\ .$$
In the real domain the "general power function" $a^b$ is defined for $a>0$ by $a^b:=\exp(b\,\log a)$. Transferring this principle into the complex world one arrives at $$z^w:=\exp(w\ \log z)=\exp\bigl((u+iv)(\log r+i\phi +2k\pi i)\bigr)\qquad (r>0)\ ,$$ resp. at $$z^w:=\exp(w\ {\rm Log} z)=\exp\bigl((u+iv)(\log r + i\phi)\bigr)\qquad(r>0, \ |\phi|<\pi)\ .$$
It seems that the OP is interested in the first of the two interpretations. Therefore let's look at the example $i^i$. As $i=e^{i\pi/2}$ one has $r=1$, $\phi={\pi\over2}$. Therefore we get $$i^i=\exp\bigl( (0+i)(\log 1+i{\pi\over2}+2k\pi i)\bigr)=\exp\bigl(-{\pi\over2}-2k\pi \bigr)\ ,$$ where this is to be interpreted as the resulting set when $k$ runs through ${\mathbb Z}$. We see that this set $\{c_k\}_{k\in{\mathbb Z}}$ consists of positive real numbers. For $k\leq-1$ the $c_k$ lies outside the interval $[0,4\pi]$; but for all $k\geq0$ one has $c_k\in[0,4\pi]$, so that, yes: there are infinitely many values of $i^i$ in the given interval.
A number $x$ of $\mathbb{C} $ is a primitive $n^{th}$ root if $x^n - 1 = 0$, but $x^j -1 \not= 0$ for any $0 \leq j < n$. If we for each natural number $n$ finds the primitive roots of $x^n -1 = 0$ and collects them for all such $n$, then we find all roots once. In number, the primitive $n^{th}$ roots are $\varphi (n)$ where the function is Eulers phi function, counting how many numbers in the interval $[0, n)$ being relative prime to $n$, where $j$ is relative prime to $n$ if $gcd (n,j)=1$ (greatest common divisor).For such $j$ we have a root \begin{equation}\nonumber e^{i\frac{ 2 \pi }{n }j}. \end{equation} If $z$ is a complex number, $z$ can be written $z = |z| e^{2 \pi i( \gamma+ m )} = (|z|^{1/n} e^{2 \pi i (\gamma+ m ) /n})^n$. Therefore an equation $x^n - z = 0$ has an expression \begin{equation}\nonumber \Big(\frac{x}{|z|^{1/n} e^{2 \pi i (\gamma + m)/n}} \Big)^n - 1 = 0 \end{equation} From this, the $n^{\text{th }}$ roots are \begin{equation}\nonumber x =|z|^{1/n}e^{i \frac{2 \pi }{n }\big(\gamma + j + m\big) } \end{equation} If $|z|\not =1$, roots belonging to different $n$ will be different. If $|z|=1$, $m+j$ must be prime to $n$. In that case, we get the contribution from $n$ to be the roots \begin{equation}\nonumber x =|z|^{1/n}e^{i \frac{2 \pi }{n }\big(\gamma + j + m\big) } \qquad \text{where } \gcd(n, j+m)=1. \end{equation} or, what is the same, \begin{equation}\nonumber x =|z|^{1/n}e^{i \frac{2 \pi }{n }\big(\gamma +k\big) } \qquad \text{where } \gcd(n, k)=1, \qquad 1 \leq k < n. \end{equation} For example, when $z$ equals $i^i$, we have a real number, so take $\gamma = 0$ and the real part is $\exp (- \pi /2)$. This should give the $n^{th}$ roots \begin{equation}\nonumber x = e^{-\frac{\pi }{2 n } +i \frac{2 \pi }{n } k} . \qquad \text{where } 1 \leq k < n. \end{equation} When $z$ equals $i^{1/e}$, rewritten ($(e^{i \pi /2})^{1/e})=e^{i \pi /(2 e)}$, $|z|=1$ and $\gamma $ from above is $\pi /(2 e)+ 2 \pi m $. Then the roots which are $n^{th}$ roots but not $k^{th}$ roots for $k < n$ are \begin{equation}\nonumber x = e^{i \frac{2 \pi }{n }\big(\frac{1}{4 e } + k\big) } \qquad \text{where } \gcd(n,k) = 1. \end{equation} Collecting together for all natural numbers $n$ should give all possible roots in the respective cases.