Number of balls touching one ball at the center. Let $B$ be a ball of radius one in $\mathbb{R}^d$. I'm looking for references about the following problem :
How many disjoint balls $B_i$ of radius one is it possible to put in contact (tangential contact) to the ball $B$ ?
For example, if $d=2$, we can put a maximum of $6$ balls touching $B$ (each $B_i$ will then be in contact with $B$ + two other neighbor balls, see picture below).
I found out that this problem is linked to the question of "compact arrangement of balls" arising in physics and chemistry (especially in crystallography), but I don't know the classic references for a mathematical approach. Does this problem have a mathematical name ? Do you have references to share ?
Any help is welcomed, please feel free to re-tag as appropriate.


It is called the kissing number.
