VSI electromagnetic fields
  A $p$-form $F$ is VSI (i.e., all its scalar invariants of arbitrary order vanish) in a $n$-dimensional spacetime if and only if it is of type N, its multiple null direction $l$ is "degenerate Kundt", and $\pounds_{l}F=0$. This recent result is reviewed in the present contribution and its main consequences are summarized. In particular, a subset of VSI Maxwell fields possesses a universal property, i.e., they also solve (virtually) any generalized (non-linear and with higher derivatives) electrodynamics, possibly also coupled to Einstein's gravity. 
Marcello Ortaggio, Vojt\v{e}ch Pravda
gr-qc