  Recently, Kozik, Ferrero and Georges have discovered numerically that for a
family of fundamental models of interacting fermions, the self-energy
$\Sigma[G]$ is a multi-valued functional of the fully dressed single-particle
propagator G, and that the skeleton diagrammatic series $\Sigma_{\rm bold}[G]$
converges to the wrong branch above a critical interaction strength. We
consider the zero space-time dimensional case, where the same mathematical
phenomena appear from elementary algebra. We also find a similar phenomenology
for the fully bold formalism built on fully dressed single-particle propagator
and pair propagator.
