Critical Radius for Infinite Dimensional Sphere Packing Hello. I'd like to consider the open unit ball in an infinite dimensional Hilbert space and ask when can we fit infinitely many open balls of radius $r<1$ inside.
For example, when $r=1/(1+\sqrt2)$, we can pick an orthonormal basis $(x_1,...)$ for our Hilbert space and put the centers of the balls at $(1-r)x_i = \sqrt2/(1+\sqrt2)x_i$ for each $i$. The distance between any two centers is thus $\sqrt2/(1+\sqrt2)\sqrt2 = 2r$, so indeed the balls just kiss each other.
Can we fit any larger balls? What is the critical radius $r_\infty$ such that for $r>r_\infty$ we may only fit finitely many balls of radius $r$, but for smaller $r$ we may fit infinitely many?
 A: Your value of $r$ is the best.
Equivalently, $\rho=\sqrt 2$, where $\rho$ is the sup, over all infinite sequence $(x_i)$ in the unit ball of a Hilbert space, of $\inf_{i\neq j} |x_i-x_j|$.
Here is a proof, by contradiction. Assume that $\rho>\sqrt 2$ and pick a sequence $(x_i)$ such that $\inf_{i\neq j} |x_i-x_j|$ is almost $\rho$. Take $e$ the unit vector $x_1/|x_1|$. Then from the inequality $|x_i-x_1|>\sqrt 2$ we get that $\langle x_i,e\rangle<0$, and even that $\langle x_i,e\rangle<-\delta$ for some positive $\delta$ depending on $\rho$ only. In particular, every element in the sequence $(x_2,x_3,...)$ belongs to the ball of radius $1-\epsilon$ around $-\delta e$ for some $\epsilon>0$ depending on $\rho$ only. This implies that $\inf_{i\neq j >1} |x_i-x_j| \leq (1-\epsilon)\rho$. But $\inf_{i\neq j} |x_i-x_j|$ was arbitraly close to $\rho$. We thus get $\rho  \leq (1-\epsilon)\rho$, a contradiction.
A: The optimality of your configuration can be shown as a plain consequence of the Kirszbraun theorem.
(I happened to ask myself this problem too, and eventually added this short section in a wiki article, thinking that it could be useful one day --not completely true, since your question has been already answered by Mikael de la Salle). 
