A (simple) arc in finite projective geometry is a set of points which satisfies, in an intuitive way, a feature of curved figures in continuous geometries. Loosely speaking, they are sets of points that are far from "line-like" in a plane or far from "plane-like" in a three-dimensional space. In this finite setting it is typical to include the number of points in the set in the name, so these simple arcs are called k -arcs. An important generalization of k-arcs, also referred to as arcs in the literature, is the (k, d)-arcs.

k-arcs in a projective plane

In a finite projective plane π (not necessarily Desarguesian) a set A of k (k ≥ 3) points such that no three points of A are collinear (on a line) is called a k - arc . If the plane π has order q then k ≤ q + 2 , however the maximum value of k can only be achieved if q is even. In a plane of order q, a (q + 1) -arc is called an oval and, if q is even, a (q + 2) -arc is called a hyperoval. Every conic in the Desarguesian projective plane PG(2,q), i.e., the set of zeros of an irreducible homogeneous quadratic equation, is an oval. A celebrated result of Beniamino Segre states that when q is odd, every (q + 1) -arc in PG(2,q) is a conic (Segre's theorem). This is one of the pioneering results in finite geometry. If q is even and A is a (q + 1) -arc in π, then it can be shown via combinatorial arguments that there must exist a unique point in π (called the nucleus of A) such that the union of A and this point is a (q + 2)-arc. Thus, every oval can be uniquely extended to a hyperoval in a finite projective plane of even order. A k-arc which can not be extended to a larger arc is called a complete arc. In the Desarguesian projective planes, PG(2,q), no q-arc is complete, so they may all be extended to ovals.

k-arcs in a projective space

In the finite projective space PG( n, q ) with n ≥ 3 , a set A of k ≥ n + 1 points such that no n + 1 points lie in a common hyperplane is called a (spatial) k -arc. This definition generalizes the definition of a k-arc in a plane (where n = 2 ).

(

k, d

)-arcs in a projective plane A ( k, d )-arc ( k, d > 1 ) in a finite projective plane π (not necessarily Desarguesian) is a set, A of k points of π such that each line intersects A in at most d points, and there is at least one line that does intersect A in d points. A ( k, 2 )-arc is a k-arc and may be referred to as simply an arc if the size is not a concern. The number of points k of a ( k, d )-arc A in a projective plane of order q is at most qd + d − q . When equality occurs, one calls A a maximal arc. Hyperovals are maximal arcs. Complete arcs need not be maximal arcs.