A 1-planar graph is a graph that can be drawn in the Euclidean plane such that each edge crosses at most one edge. An independent crossing (IC)-planar graph is a 1-planar graph satisfying the condition that two pairs of crossing edges have no common end-vertices. It is shown in this paper that every IC-planar graph is 6-choosable.

For a planar graph G, let $\Delta(G)$, $g(G)$, and $\lambda(G;p,q)$ denote, respectively, its maximum degree, girth, and $L(p,q)$-labeling number. We prove that (1) $\lambda(G;p,q)\le (2q-1)\Delta(G)+4p+4q-4$ if $g(G)\ge 7$; (2) $\lambda(G;p,q)\le (2q-1)\Delta(G)+6p+12q-9$ if $g(G)\ge 6$; (3) $\lambda(G;p,q)\le (2q-1)\Delta(G)+6p+24q-15$ if $g(G)\ge 5$. These bounds have consequences on conjectures by Wegner [Graphs with Given Diameter and a Coloring Problem, preprint, University of Dortmund, Dortmund, Germany, 1977] and Griggs and Yeh [SIAM J. Discrete Math., 5 (1992), pp. 586--595].

Let G be a planar graph without two triangles sharing a common vertex. We prove that (1) G is 4-choosable and (2) G is edge-$(\Delta(G)+1)$-choosable when its maximum degree $\Delta(G)\ne 5$.

A graph is called star extremal if its fractional chromatic number is equal to its circular chromatic number (also known as the star chromatic number). We prove that members of a certain family of circulant graphs are star extremal. The result generalizes some known theorems of Sidorenko [Discrete Math., 91 (1991), pp. 215--217] and Gao and Zhu [Discrete Math., 152 (1996), pp. 147--156]. We show relations between circulant graphs and distance graphs and discuss their star extremality. Furthermore, we give counterexamples to two conjectures of Collins [SIAM J. Discrete Math., 11 (1998), pp. 330--339] on asymptotic independence ratios of circulant graphs.

Roughly speaking partial degrees are equivalence classes of partial objects under a certain notion of relative recursiveness. To make this notion precise we have to state explicitly (1) what these partial objects are; (2) how to define a suitable reduction procedure. For example, when the type of these objects is restricted to one, we may include all possible partial functions from natural numbers to natural numbers as basic objects and the reduction procedure could be enumeration, weak Turing, or Turing reducibility as expounded in Sasso [4]. As we climb up the ladder of types, we see that the usual definitions of relative recursiveness, equivalent in the context of type-1 total objects and functions, may be extended to partial objects and functions in quite different ways. First such generalization was initiated by Kleene [2]. He considers partial functions with total objects as arguments. However his theory suffers the lack of transitivity, i.e. we may not obtain a recursive function when we substitute a recursive function into a recursive function. Although Kleene's theory provides a nice background for the study of total higher type objects, it would be unsatisfactory when partial higher type objects are being investigated. In this paper we choose the hierarchy of hereditarily consistent objects over ω as our universe of discourse so that Sasso's objects are exactly those at the type-1 level. Following Kleene's fashion we define relative recursiveness via schemes and indices. Yet in our theory, substitution will preserve recursiveness, which makes a degree theory of partial higher type objects possible. The final result will be a natural extension of Sasso's Turing reducibility. Due to the abstract nature of these objects we do not know much about their behaviour except at the very low types. Here we pay our attention mainly to type-2 objects. In §2 we formulate basic notions and give an outline of our recursion theory of partial higher type objects. In §3 we introduce the definitions of singular degrees and ω-consistent degrees which are two important classes of type-2 objects that we are most interested in.