  We show that the Duffin-Kemmer-Petiau equation, minimally coupled to an
Abelian gauge field, can be regarded as a matrix equation for the gauge
potential produced internally from the matter fields. This can be solved as a
rational expression in terms of currents bilinear in the matter wavefunction,
together with a similar expression for the field strength tensor, thus
providing a gauge invariant formulation of the self-interacting DKP equations.
We give the derivation of this result for the 5 component DKP system, by
analogy with the Dirac equation case. To this end, we establish the algebraic
structure of the set of bilinear currents, and the properties of the minimal
generating set, which consists of two scalars and two four-vectors, together
with a single quadratic constraint.
