A k-colouring of a graph G is a mapping c from the set of vertices of G to the set {1, . . . , k} of colours such that adjacent vertices receive distinct colours. Such a k-colouring is called acyclic, if for every two distinct colours i and j, the subgraph induced by all the edges linking a vertex coloured with i and a vertex coloured with j is acyclic. In other words, every cycle in G has at l...

A signed graph is a simple with two types of edges: positive and negative. homomorphism from G to another H mapping \(\varphi : V(G) \rightarrow V(H)\) that preserves vertex adjacencies balance closed walks (the the parity number negative edges). The chromatic \(\chi _s(G)\) order smallest such there H.

A proper vertex coloring of a simple graph is k-forested if the graph induced by the vertices of any two color classes is a forest with maximum degree less than k. A graph is k-forested qchoosable if for a given list of q colors associated with each vertex v, there exists a k-forested coloring of G such that each vertex receives a color from its own list. In this paper, we prove that the k-fore...

Improper choosability of planar graphs has been widely studied. In particular, Škrekovski investigated the smallest integer gk such that every planar graph of girth at least gk is k-improper 2-choosable. He proved [9] that 6 ≤ g1 ≤ 9; 5 ≤ g2 ≤ 7; 5 ≤ g3 ≤ 6 and ∀k ≥ 4, gk = 5. In this paper, we study the greatest real M(k, l) such that every graph of maximum average degree less than M(k, l) is ...