  Consider a critical nearest neighbor branching random walk on the
$d$-dimensional integer lattice initiated by a single particle at the origin.
Let $G_{n}$ be the event that the branching random walk survives to generation
$n$. We obtain limit theorems conditional on the event $G_{n}$ for a variety of
occupation statistics: (1) Let $V_{n}$ be the maximal number of particles at a
single site at time $n$. If the offspring distribution has finite $\alpha$th
moment for some integer $\alpha\geq 2$, then in dimensions 3 and higher,
$V_n=O_p(n^{1/\alpha})$; and if the offspring distribution has an exponentially
decaying tail, then $V_n=O_p(\log n)$ in dimensions 3 and higher, and
$V_n=O_p((\log n)^2)$ in dimension 2. Furthermore, if the offspring
distribution is non-degenerate then $P(V_n\geq \delta \log n | G_{n})\to 1$ for
some $\delta >0$. (2) Let $M_{n} (j)$ be the number of multiplicity-$j$ sites
in the $n$th generation, that is, sites occupied by exactly $j$ particles. In
dimensions 3 and higher, the random variables $M_{n} (j)/n$ converge jointly to
multiples of an exponential random variable. (3) In dimension 2, the number of
particles at a "typical" site (that is, at the location of a randomly chosen
particle of the $n$th generation) is of order $O_p(\log n)$, and the number of
occupied sites is $O_p(n/\log n)$.
