Let $\mathcal A$ and $\mathcal B$ be graph algebras. In this paper we present the notion of an ideal in a graph algebra and prove that an ideal extension of $\mathcal A$ by $\mathcal B$ always exists. We describe (up to isomorphism) all such extensions.
In the present paper we introduce the notion of an ideal of a partial monounary algebra. Further, for an ideal $(I,f_I)$ of a partial monounary algebra $(A,f_A)$ we define the quotient partial monounary algebra $(A,f_A)/(I,f_I)$. Let $(X,f_X)$, $(Y,f_Y)$ be partial monounary algebras. We describe all partial monounary algebras $(P,f_P)$ such that $(X,f_X)$ is an ideal of $(P,f_P)$ and $(P,f_P)/(X,f_X)$ is isomorphic to $(Y,f_Y)$.
For a subalgebra ${\mathcal B}$ of a partial monounary algebra ${\mathcal A}$ we define the quotient partial monounary algebra ${\mathcal A}/{\mathcal B}$. Let ${\mathcal B}$, ${\mathcal C}$ be partial monounary algebras. In this paper we give a construction of all partial monounary algebras ${\mathcal A}$ such that ${\mathcal B}$ is a subalgebra of ${\mathcal A}$ and ${\mathcal C}\cong {\mathcal A}/{\mathcal B}$.