On Sierpiński packing chromatic number and recognition of Sierpiński products

Abstract: The Sierpi\'{n}ski product $G \otimes <em>f H$ of graphs $G$ and $H$ with respect to a function $f \colon V(G)\rightarrow V(H)$ has the vertex set $V(G)\times V(H)$. For every $g\in V(G)$ it contains a disjoint copy $gH$ of $H$, and for every edge $gg&#39;$ of $G$ there is the edge $(g,f(g&#39;))(g&#39;,f(g))$ between $gH$ and $g&#39;H$. In this paper, the Sierpi\'{n}ski packing chromatic number is defined as the minimum of $\chi</em>{\rho}(G\otimes <em>f H)$ over all functions $f$, where $\chi</em>{\rho}(X)$ is the packing chromatic number of $X$. The upper Sierpi\'{n}ski packing chromatic number is analogously defined as the maximum corresponding value. The (upper) Sierpi\'{n}ski packing chromatic number is determined for all Sierpi\'{n}ski product graphs whose both factors are complete. Sierpi\'{n}ski product graphs whose factors are paths or stars are also studied. Their Sierpi\'{n}ski packing chromatic number is always $3$, while their upper Sierpi\'{n}ski packing chromatic number is bounded from below and above. It is also proved that for a given graph $G$, it can be checked in polynomial time whether $G$ has a representation as a Sierpi\'{n}ski product graphs both factors of which being trees.