The notion that graphs with very few edges tend to have large pebbling
number and graphs with very many edges tend to have small pebbling
number can be made precise as follows.
Let
$\mathbf{G}_{n,p}$
be the random graph model in which each of the
${n \choose 2}$
possible edges of a random graph having
$n$
vertices appears independently with probability
$p$.
For functions
$f$
and
$g$
on the natural numbers we write that
$f \ll g$
(or
$g \gg f$)
when
$f/g \rightarrow 0$
as
$n \rightarrow \infty$.
Let
$o(G)=\{f | f \ll g \}$
and define
$O(g)$
(resp.,
$\Omega(G)$)
to be the set of functions
$f$
for which there are constants
$c,N$
such that
$f(n) \leq cg(n)$
(resp.,
$f(n) \geq cg(n)$)
whenever
$n > N$.
Finally, let
$\Theta(g) = O(g) \cap \Omega(g)$.
Let
$\mathbf{P}$
be a property of graphs and consider the probability
${\rm Pr}[\mathbf{P}]={\rm Pr}_p[\mathbf{P}]$
that the random graph
$\mathbf{G}_{n,p}$
has
$\mathbf{P}$.
For large
$p$
it may be that
${\rm Pr}[\mathbf{P}] \rightarrow 1$
as
$n \rightarrow \infty$,
and for small
$p$
it may be that
${\rm Pr}[\mathbf{P}] \rightarrow 0$
as
$n \rightarrow \infty$.
More precisely, define the
threshold
of
$\mathbf{P}$,
$t(\mathbf{P})$,
to be the set of functions
$t$
for which
$p \gg t$
implies that
${\rm Pr}[\mathbf{P}] \rightarrow 0$
as
$n \rightarrow \infty$.
It is not clear that such thresholds exist for arbitrary
$\mathbf{P}$.
However, we observe that Class 0 is a monotone property (adding edges
to a Class 0 graph maintains the property), and a theorem of Bollobas
and Thomason
[BoTh]
states that
$t(\mathbf{P})$
is nonempty for every
monotone
$\mathbf{P}$.
It is well known
[ErRe]
that
$t({\rm connected}) = \Theta(\log n/n)$,
and since connectedness is required for Class 0, we see that
$t({\rm Class}$ $0) \subset \Omega(\log n/n)$.
In
[CHH]
it is noted that
$t({\rm Class}$ $0) \subset O(1)$.
In
[CHKT]
the
Diameter-Connectivity Theorem
was used to prove the following result.