Perturbative expansions in quantum field theory diverge for at least two reasons:
the number of Feynman diagrams increases dramatically with the loop number
and the process of renormalization may make the contribution of some diagrams large.
We give an example of the second problem, from an ultra-violent renormalon of
$\phi^3$ theory in 6 dimensions, where we can compute to very high loop-order.
Taming this renormalon involves recent work on resurgence.
This challenge is much more demanding than the corresponding problem for Yukawa
theory in 4 dimensions.