  We prove that families of polar codes with multiple kernels over certain
symmetric channels can be viewed as polar decreasing monomial-Cartesian codes,
offering a unified treatment for such codes, over any finite field. We define
decreasing monomial-Cartesian codes as the evaluation of a set of monomials
closed under divisibility over a Cartesian product. Polar decreasing
monomial-Cartesian codes are decreasing monomial-Cartesian codes whose sets of
monomials are closed respect a partial order inspired by the recent work of
Bardet, Dragoi, Otmani, and Tillich ["Algebraic properties of polar codes from
a new polynomial formalism," 2016 IEEE International Symposium on Information
Theory (ISIT)]. Extending the main theorem of Mori and Tanaka ["Source and
Channel Polarization Over Finite Fields and Reed-Solomon Matrices," in IEEE
Transactions on Information Theory, vol. 60, no. 5, pp. 2720--2736, May 2014],
we prove that any sequence of invertible matrices over an arbitrary field
satisfying certain conditions polarizes any symmetric over the field channel.
In addition, we prove that the dual of a decreasing monomial-Cartesian code is
monomially equivalent to a decreasing monomial-Cartesian code. Defining the
minimal generating set for a set of monomials, we use it to describe the
length, dimension and minimum distance of a decreasing monomial-Cartesian code.
