# Mean square of x-component of uniformly distributed circle points

Recently I was looking at a paper where the velocity auto-correlation function $$C(t) = \langle v_x(t) v_x(0) \rangle = \langle \cos\theta(t), \cos \theta(0) \rangle$$ was being considered for a number of point particles with velocities distributed uniformly on $$S^1$$ at time zero (here $$\langle \cdot \rangle$$ denotes an average over initial conditions). In the above, $$\theta(t)$$ is the angle w.r.t to the horizontal at time $$t$$. In their plot of $$C(t)$$ vs. $$t$$ I noticed that $$C(0) \ne 1/2$$ (there were multiple plots). However I don't see this when trying to generate a uniform velocity distribution so I assume I am doing something wrong here.

I generate velocities uniformly on $$S^1$$ as follows:

    N <- 10^4                                          # number of samples
r <- runif(N, 0, 1)                                # uniform radii in [0,1]
theta <- runif(N, -pi, pi)                         # uniform angles
P <- cbind(sqrt(r)*cos(theta), sqrt(r)*sin(theta)) # uniform points in the unit disk
L <- sqrt(P[,1]*P[,1] + P[,2]*P[,2])
V <- P/L


Another method I've seen is:

X1 <- rnorm(N, 0, 1)
X2 <- rnorm(N, 0, 1)
R  <- sqrt(X1*X1 + X2*X2)
V  <- matrix(rbind(X1/R, X2/R), N, 2, byrow=TRUE)


Or using the pracma package:

pracma::rands(N, r=1.0, N=1.0)


Firstly, can someone confirm that these are appropriate methods for generating points uniformly on the unit circle?

In all cases mean(V[,1] * V[,1]) returns $$\approx 1/2$$.

Moreover if $$\Theta \sim \mathcal{U}[-\pi, \pi]$$ and $$V = \cos^2 \Theta$$ then it has the pdf $$f_V(x) = \dfrac{-2}{\pi} \dfrac{d}{dx} (\sin^{-1} \sqrt{x}) = \dfrac{-1}{\sqrt{x(1-x)}}$$ which has mean value $$\int_0^1 x f_V(x) dx = 1/2.$$

Is this correct?

Edit:

The issue with the final calculation is that uniform points on the circle are not formed by simply taking the cosine and sine of uniformly distributed angles, so it must be incorrect..

Integration over initial condition also gives $$1/2$$. $$C(0) = \left< \cos \theta, \cos \theta\right> = \frac{\int_{-\pi}^{\pi} \cos^2 \theta \, d\theta }{ \int_{-\pi}^{\pi} \, d\theta} = \frac{1}{2}$$
It could be that in the paper it is normalized by $$C(0)$$.
• @algae both of your codes do not sample from $S^1$. I think the simple way is to take cosine and sine of runif(N, -pi, pi). May 24 '20 at 8:40
• AFAIK that is the classic mistake that is made when sampling points uniformly on $S^1$. The first code is based on the paper here. May 24 '20 at 8:51