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For f:R2→R, let ΓR(f) be a two‐dimensional algebraically defined graph, that is, a bipartite graph where each partite set is a copy of R2 and two vertices (a,a2) and [x,x2] are adjacent if and only if a2+x2=f(a,x). It is known that ΓR(XY) has girth 6 and can be extended to the point‐line incidence graph of the classical real projective plane. However, it was unknown whether there exists f∈R[X,Y] such that ΓR(f) has girth 6 and is nonisomorphic to ΓR(XY). This paper answers this question affirmatively and thus provides a construction of a nonclassical real projective plane. This paper also studies the diameter and girth of ΓR(f) for families of bivariate functions f. [ABSTRACT FROM AUTHOR]

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Grids with blocked and unblocked cells are often used to represent terrain in robotics and video games. However, paths formed by grid edges can be longer than true shortest paths in the terrain since their headings are artificially constrained. We present two new correct and complete any-angle path-planning algorithms that avoid this shortcoming. Basic Theta* and Angle-Propagation Theta* are both variants of A* that propagate information along grid edges without constraining paths to grid edges. Basic Theta* is simple to understand and implement, fast and finds short paths. However, it is not guaranteed to find true shortest paths. Angle-Propagation Theta* achieves a better worst-case complexity per vertex expansion than Basic Theta* by propagating angle ranges when it expands vertices, but is more complex, not as fast and finds slightly longer paths. We refer to Basic Theta* and Angle-Propagation Theta* collectively as Theta*. Theta* has unique properties, which we analyze in detail. We show experimentally that it finds shorter paths than both A* with post-smoothed paths and Field D* (the only other version of A* we know of that propagates information along grid edges without constraining paths to grid edges) with a runtime comparable to that of A* on grids. Finally, we extend Theta* to grids that contain unblocked cells with non-uniform-raversal costs and introduce variants of Theta* which provide different tradeoffs between path length and runtime. [ABSTRACT FROM AUTHOR]