We consider stochastic mean field games for which the state space is a network. In the ergodic case, they are described by a system coupling a Hamilton-Jacobi-Bellman equation and a Fokker-Planck equation, whose unknowns are the invariant measure $ m $, a value function $ u $, and the ergodic constant $ \rho $. The function $ u $ is continuous and satisfies general Kirchhoff conditions at the vertices. The invariant measure $ m $ satisfies dual transmission conditions: in particular, $ m $ is discontinuous across the vertices in general, and the values of $ m $ on each side of the vertices satisfy special compatibility conditions. Existence and uniqueness are proven under suitable assumptions.
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Figure 1. Left: the network $ \Gamma $ in which the edges are oriented toward the vertex with larger index ($ 4 $ vertices and $ 4 $ edges). Right: a new network $ \tilde \Gamma $ obtained by adding an artificial vertex ($ 5 $ vertices and $ 5 $ edges): the oriented edges sharing a given vertex $ \nu $ either have all their starting point equal $ \nu $, or have all their terminal point equal $ \nu $
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Left: the network