Circle of a sphere Circle of a sphere A circle of a sphere is a circle that lies on a sphere. Such a circle can be formed as the intersection of a sphere and a plane, or of two spheres. A circle on a sphere whose plane passes through the center of the sphere is called a great circle; otherwise it is a small circle. Circles of a sphere have radius less than or equal to the sphere radius, with equality when the circle is a great circle. In the geographic coordinate system on a globe, the parallels of latitude are such circles, with
Straight-line grammar minimal size that generates a given string is called the smallest grammar problem. Straight-line grammars (more precisely: straight-line context-free string grammars) can be generalized to "Straight-line context-free tree grammars". The latter can be used conveniently to compress trees. A context-free grammar "G" is an SLG if: 1. for every non-terminal "N", there is at most one production rule that has "N" as its left-hand side, and 2. the directed graph "G"=<"V","E">, defined by "V" being the set of non-terminals and ("A","B") ∈ "E" whenever "B" appears at the right-hand side of a production rule for "A", is acyclic. A mathematical