Multi-linear iterative K-Sigma-semialgebras

We consider $K$-semialgebras for a commutative semiring $K$ that are at the same time $\Sigma$-algebras and satisfy certain linearity conditions. When each finite system of guarded polynomial fixed point equations has a unique solution over such an algebra, then we call it an iterative multi-linear $K$-$\Sigma$-semialgebra. Examples of such algebras include the algebras of $\Sigma$-tree series over an alphabet $A$ with coefficients in $K$, and the algebra of all rational tree series. We show that for many commutative semirings $K$, the rational $\Sigma$-tree series over $A$ with coefficients in $K$ form the free multi-linear iterative $K$-$\Sigma$-semialgebra on $A$.