# Problem 2.7

A First Course in String Theory

.

2.7 A more general construction for cones?

Consider the $\displaystyle{(x, y)}$ plane and the complex coordinate $\displaystyle{z = x + iy}$. We have seen that the identification $\displaystyle{z \sim e^{\frac{2 \pi i}{N}} z}$, with $\displaystyle{N}$ an integer greater than two, can be used to construct a cone.

Examine now the identification

$\displaystyle{z \sim e^{2 \pi i \frac{M}{N}} z, ~~~ N > M \ge 2,}$

where $\displaystyle{M}$ and $\displaystyle{N}$ are relatively prime integers (their greatest common divisor is one).

Determine a fundamental domain for identification.

Given two relatively prime numbers a and b, there exists integers m and n such that $\displaystyle{m a + n b = 1}$.

~~~

[guess]

Since M and N are relatively prime numbers, there exists integers m and n such that $\displaystyle{m M + n N = 1}$.

So

$\displaystyle{\frac{M}{N} m = \frac{1 - nN}{N}}$.

Therefore,

$\displaystyle{\left[e^{2 \pi i \frac{M}{N}}\right]^m = e^{2 \pi i \frac{1 - nN}{N}}}$ for some integers m and n.

As a result, a fundamental domain is provided by the points z that satisfy $\displaystyle{0 \le \arg(z) < 2 \pi \frac{1}{N}}$.

[guess]

— Me@2021-03-09 04:58:02 PM

.

.