  The first part of this paper deals with electrical networks and symplectic
reductions. We consider two operations on electrical networks (the "trace map"
and the "gluing map") and show that they correspond to symplectic reductions.
We also give several general properties about symplectic reductions, in
particular we study the singularities of symplectic reductions when considered
as rational maps on Lagrangian Grassmannians. This is motivated by [23] where a
renormalization map was introduced in order to describe the spectral properties
of self-similar lattices. In this text, we show that this renormalization map
can be expressed in terms of symplectic reductions and that some of its key
properties are direct consequences of general properties of symplectic
reductions (and the singularities of the symplectic reduction play an important
role in relation with the spectral properties of our operator). We also present
new examples where we can compute the renormalization map.
