An exact classical field theory for nonlinear quantum equations is presented herein. It has been applied recently to a nonlinear Schrödinger equation, and it is shown herein to hold also for a nonlinear generalization of the Klein-Gordon equation. These generalizations were carried by introducing nonlinear terms, characterized by exponents depending on an index q, in such a way that the standard, linear equations, are recovered in the limit q → 1. The main characteristic of this field theory consists on the fact that besides the usual

$\Psi (\vec{x},t)$
Ψ(x,t)⁠, a new field
$\Phi (\vec{x},t)$
Φ(x,t)
needs to be introduced in the Lagrangian, as well. The field
$\Phi (\vec{x},t)$
Φ(x,t)
, which is defined by means of an additional equation, becomes
$\Psi ^{*}(\vec{x},t)$
Ψ*(x,t)
only when q → 1. The solutions for the fields
$\Psi (\vec{x},t)$
Ψ(x,t)
and
$\Phi (\vec{x},t)$
Φ(x,t)
are found herein, being expressed in terms of a q-plane wave; moreover, both field equations lead to the relation E2 = p2c2 + m2c4, for all values of q. The fact that such a classical field theory works well for two very distinct nonlinear quantum equations, namely, the Schrödinger and Klein-Gordon ones, suggests that this procedure should be appropriate for a wider class nonlinear equations. It is shown that the standard global gauge invariance is broken as a consequence of the nonlinearity.

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