Promise Systems of Equations over Magmas with Identity and over Algebras in Congruence Modular Varieties

We study the computational complexity of solving promise systems of equations over finite algebras. Given two algebras $\mathbf{A}$ and $\mathbf{B}$ with a homomorphism from $\mathbf{A}$ to $\mathbf{B}$, the promise system of equations problem is to determine if an input system of equations has a solution in $\mathbf{A}$ or not even in $\mathbf{B}$. We generalize the results of Larrauri, Mottet, and Živný [ACM ToCL'26] to obtain a $\mathbf{P}-\mathbf{NP}$-hard dichotomy result for promise systems of equations over a class of algebras which contains all monoids, and a dichotomy result for promise systems of equations over algebras in a congruence modular variety. We then consider the metaproblem for promise systems of equations over algebras in a congruence modular variety: given finite algebras $\mathbf{A}$ and $\mathbf{B}$ such that $\mathbf{A}$ is in a congruence modular variety, we show there is a quasi-polynomial time algorithm for determining whether or not the associated promise system of equations problem is in $\mathbf{P}$.