Limit to Infinity of Summation of Euler Phi Function over Square/Proof 2
Theorem
- $\ds \lim_{n \mathop \to \infty} \dfrac {\map \Phi n} {n^2} = \dfrac 3 {\pi^2}$
where:
- $\map \Phi n = \ds \sum_{k \mathop = 1}^n \map \phi k$
- $\map \phi k$ is the Euler $\phi$ function of $k$.
Numerically, this evaluates to:
- $\dfrac 3 {\pi^2} \approx 0 \cdotp 30396 35509 \ldots$
Proof
We find the probability that the two random numbers not less than $n$ is coprime.
We pick randomly $x, y \in \set {1, \dots, n}$.
There are $n^2$ ways to do this.
There are three cases:
If $x > y$, for each $x$, there are $\map \phi x$ values of $y$ so that $x \perp y$.
Since $1 \le x \le n$, there are a total of $\map \phi 1 + \dots + \map \phi n = \map \Phi n$ valid values of $y$.
If $x < y$, the result is similar: there are $\map \Phi n$ valid values of $x$.
If $x = y$, $x \perp y$ only if $x = y = 1$.
Therefore the probability that the two random numbers not less than $n$ is coprime is:
- $\dfrac {\map \Phi n + \map \Phi n + 1} {n^2}$
Taking limit to infinity and using Probability of Two Random Integers having no Common Divisor:
\(\ds \frac 6 {\pi^2}\) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \frac {\map \Phi n + \map \Phi n + 1} {n^2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \frac {2 \map \Phi n} {n^2} + \lim_{n \mathop \to \infty} \frac 1 {n^2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \frac {2 \map \Phi n} {n^2}\) | Basic Null Sequences | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds \lim_{n \mathop \to \infty} \frac {\map \Phi n} {n^2}\) | \(=\) | \(\ds \frac 3 {\pi^2}\) |
$\blacksquare$