This paper discusses path controlled grammars { context-free gram- mars with a root-to-leaf path in their derivation trees restricted by a control language. First, it investigates the impact of erasing rules on the generative power of path controlled grammars. Then, it establishes two Chomsky-like normal forms for path controlled grammars - the first allows unit rules, the second allows just one erasing rule.
The concept of [r, s, t]-colourings was recently introduced by Hackmann, Kemnitz and Marangio [3] as follows: Given non-negative integers r, s and t, an [r, s, t]-colouring of a graph G = (V(G), E(G)) is a mapping c from V(G) ∪ E(G) to the colour set {1, 2,..., k} such that c(vi) - c(vj) ≥ r for every two adjacent vertices vi, vj, c(ei) - c(ej) ≥ s for every two adjacent edges ei, ej, and c(vi) - c(ej) ≥ t for all pairs of incident vertices and edges, respectively. The [r, s, t]-chromatic number Xr,s,t(G) of G is defined to be the minimum k such that G admits an [r, s, t]-colouring. In this paper, we determine the [r, s, t]-chromatic number for paths.
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