Quasi-projective monounary algebras

Abstract

Wu and Jans introduced quasi-projective modules where they say a $\cal R$ module $\cal M$ is quasi-projective if for every submodule $\cal N$, for every homomorphism $f : {\cal M} \rightarrow {\cal M}/{\cal N}$ and every epimorphism $j: {\cal M}\rightarrow {\cal M}/{\cal N}$ there is an endomorphism $\phi$ of $\cal M$ such that $\phi\circ j=f$. We say that a structure $\cal S$ is quasi-projective if for every structure $\cal T$, for every homomorphism $f : {\cal S} \rightarrow {\cal T}$ and every epimorphism $j: {\cal S}\rightarrow {\cal T}$ there is an endomorphism $\phi$ of $\cal S$ such that $\phi\circ j=f$. In 2004 D. Jakub\'ikov\'a-Studenovsk\'a defined the concept of the factor algebra denoted by ${\cal A}/{\cal B}$, where ${\cal A}$ is a monounary algebra and ${\cal B}$ is a subalgebra of $\cal A$. In this paper, we characterise the quasi-projective monounary algebras of arbitrary cardinalities for the definition of D. Jakub\'ikov\'a-Studenovsk\'a and for the second definition.