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In this video, the author claims that the equation in question has complex solutions

$z=-\frac{i\ln(2)}{2\pi n}, n\in \mathbb{Z}, n\neq 0$

Obtained by considering $1$ outside the principal argument (eg: using $2n\pi$ as it’s argument) and using a non principal complex logarithm.

Im not fully convinced that this is an valid way of solving the equation, my understanding is only the principal branch is consistent with real basis exponentiation (which is what the equation uses). [In fact, wolfram alpha says there is no solution]

Can someone elaborate on wether the video is correct or not? Preferably including the why or why not.

Thanks, -Almeida

ps: Im in highschool, so please avoid using too much advanced math, my understanding of complex analysis is VERY limited.

Almeida
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3 Answers3

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Not sure how much about complex numbers you know so bear with me. I don't like faffing around with multivalued stuff when I can help it, everything here is single-valued.

The reasoning in the video is incorrect and the comments point this out. So let's do this all properly. For $a \in \mathbb C \setminus \{0\}$ and $z \in \mathbb C$, we should define $a^z = \exp(z (\ln|a| + i \arg(a))$ for some choice of argument $\arg$ and where $\ln$ is the real logarithm. These choices of argument all differ by $2 \pi$ (because $\exp$ is periodic in $2 \pi i$). An "argument" here is a real number $\theta$ such that $z = |z| e^{i \theta}$.

If we choose $\arg 1 = 0$, we have $1^z = \exp(z \ln 1) = \exp(z \cdot 0) = 1$ for all $z \in \mathbb C$ and there are therefore no solutions.

If we instead choose $\arg 1 = 2 \pi$, we instead have $1^z = \exp(z (\log 1 + 2 \pi i)) = \exp(2 \pi i z)$. So we have the equation $\exp(2 \pi i z) = 2$. Now, we know that $\exp(z) = 1$ if and only if $z = 2 \pi i n$ for some integer $n$. We can show this by writing out $z = x + i y$ and writing $$\exp(z) = \exp(x + i y) = \exp(x) \exp(i y) = \exp(x)(\cos y + i \sin y) = 1$$ and comparing real and imaginary parts. Since $\exp(x - y) = \exp(x)\exp(-y)$, this means that if $\exp(z) = \exp(w)$, we have $\exp(z - w) = 1$ and so $z = w + 2 \pi i n$ for some integer $n$. So in the equation $\exp(2 \pi i z) = 2 = \exp(\ln(2))$, we obtain $2 \pi i z = \ln(2) + 2 \pi i n$, and so $z = n + \frac {\ln(2)} {2 \pi i} = n - \frac {i \ln(2)} {2 \pi}$ for integer $n$.

So where does this $-\frac {i \ln(2)} {2 \pi n}$ come from in the single-valued perspective? Well, if we in general pick $\arg 1 = 2 \pi k$ for some integer $k$, we will get that the solutions are $z = n - \frac {i \ln(2)} {2 \pi k}$ for integer $n$. So $-\frac {i \ln(2)} {2 \pi n}$ is a solution if you pick $\arg(z) = 2 \pi n$. They do however miss infinitely many additional solutions by not adding on integers.

If you are to mess around with multi-valued functions, we basically define $\log$ to be a set rather than a number which captures all the possible values of $\log$, ie. all the possible choices of $\arg$. So you get $\log z = \{\ln|z| + i\theta : \theta \text { satisfies } |z|e^{i \theta} = z\}$ say. So $\log 1 = \{\ldots, -2 \pi i, 0, 2 \pi i, \ldots\} = \{2 \pi i n : n \in \mathbb Z\}$. I guess you then define $1^z = \exp(z \log 1) = \{\exp(2 \pi i n z) : n \in \mathbb Z\}$, technically the image of $\exp$ under the set $z \log 1 = \{\ldots, -2\pi i z, 0, 2 \pi i z\}$. The question of whether $1^z = 2$ is then to be interpreted as which $z \in \mathbb C$ satisfy $2 \in \{\exp(2 \pi i n z) : n \in \mathbb Z\}$, in which case the answer is $z_{n, k} = n - \frac {i \ln(2)} {2 \pi k}$ for any integer $n$ and any integer $k$. Specifically, you will have $\exp(2 \pi i k z_{n, k}) = 2$. This is a very awkward sense of a "solution" though, in my opinion (maybe it's just me), but it more or less formalises what is being spoken about here.

George C
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In the below, I use the notation $\exp(x) := e^x$ for ease of reading. In brief, the answer is that the solution to $1^z = 2$ depends on what we mean by $1^z$ and what we mean by "equality".

In most contexts, $1^z = 1$ for any $z$, so clearly we have no solution to $1^z = 2$.

However, if $1^z$ is considered to be a multivalued exponentiation, then every possible value for $1^z$ can be expressed in the form $$ 1^z = [\exp\left({2k \pi i}\right)]^z = \exp\left(2 \pi ki \cdot z \right) $$ for some integer $k$. If we plug in $z = \frac{-i\ln(2)}{2 \pi n}$, we get $$ 1^z = \exp\left(2 \pi ki \cdot z \right) = \exp\left(2 \pi ki \cdot \frac{-i\ln(2)}{2 \pi n} \right) = \exp \left(\frac kn \ln(2) \right) = [\exp(\ln(2))]^{k/n} = 2^{k/n}. $$ That is, for this value of $z$, $1^z$ will be multivalued with values $2^{k/n}$ for all integers $k$. We find that the value corresponding to $k = n$ is $2^{n/n} = 2$. That is, for this particular value of $z$, one of the (infinitely many possible) values of $1^z$ is equal to $2$. So, depending on what exactly you mean by $1^z = 2$ in the context of multivalued functions, it can be said that $z = \frac{-i\ln(2)}{2\pi n}$ is a solution to this equation.

Ben Grossmann
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  • Thanks! That makes sense, I aprecciate it! – Almeida Oct 31 '23 at 18:16
  • If you dont mind answering, in what context is it interesting to consider arg(1)=$2\pi$ instead of 0? – Almeida Oct 31 '23 at 18:18
  • @Almeida I would say that for any given $z$, there are contexts where it useful to consider that function $f(w) = w^z$ as multivalued (e.g. so that you can choose a different branch cuts for different contexts). Within that context, there's nothing special about $w = 1$. – Ben Grossmann Oct 31 '23 at 20:00
  • @Almeida This may be a bit beyond your reach right now, but certain choices of argument give rise to particular "branch cuts" (lines of discontinuity in the logarithm), which can be important when computing contour integrals (making a particular choice of logarithm so that your region of integration doesn't contain a branch cut etc.). If you don't understand these words, wait a few years and you'll probably encounter it again in an undergrad math course! – George C Oct 31 '23 at 22:14
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I do not like these complex number ideas in this context.

The way I look at it is this way :

$y \neq 0$

$1 = y^0 = \frac{y}{y}$

So

$1^x = (y^0)^x = (\frac{y}{y})^x$

But $(y^0)^x = y^{0x} = y^0 = 1$

Or

$(\frac{y}{y})^x = \frac{y^x}{y^x} = 1$

I have some reasons to avoid complex numbers and some are :

  1. It is too confusing and unnecessary here.

  2. It holds very universaly , for instance for unit matrices and more advance concepts.

mick
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