  We focus our attention on the notion of intrinsic Lipschitz graphs, inside a
subclass of Carnot groups of step 2 which includes a corank 1 Carnot groups
(and so the Heisenberg groups), Free groups of step 2 and the complexified
Heisenberg group. More precisely, we prove the equivalence between intrinsic
Lipschitz map and a weak solution to a suitable non linear first order PDE
system, which generalizes Lagrangian solution in the context of Heisenberg
groups.
