Galois geometry

Galois geometry (so named after the 19th-century French mathematician Évariste Galois) is the branch of finite geometry that is concerned with algebraic and analytic geometry over a finite field (or Galois field). More narrowly, a Galois geometry may be defined as a projective space over a finite field. Objects of study include affine and projective spaces over finite fields and various structures that are contained in them. In particular, arcs, ovals, hyperovals, unitals, blocking sets, ovoids, caps, spreads and all finite analogues of structures found in non-finite geometries. Vector spaces defined over finite fields play a significant role, especially in construction methods. Although the generic notation of projective geometry is sometimes used, it is more common to denote projective spaces over finite fields by PG(n, q), where n is the "geometric" dimension (see below), and q is the order of the finite field (or Galois field) GF(q), which must be an integer that is a prime or prime power. The geometric dimension in the above notation refers to the system whereby lines are 1-dimensional, planes are 2-dimensional, points are 0-dimensional, etc. The modifier, sometimes the term projective instead of geometric is used, is necessary since this concept of dimension differs from the concept used for vector spaces (that is, the number of elements in a basis). Normally having two different concepts with the same name does not cause much difficulty in separate areas due to context, but in this subject both vector spaces and projective spaces play important roles and confusion is highly likely. The vector space concept is at times referred to as the algebraic dimension. Let V = V(n + 1, q) denote the vector space of (algebraic) dimension n + 1 defined over the finite field GF(q). The projective space PG(n, q) consists of all the positive (algebraic) dimensional vector subspaces of V. An alternate way to view the construction is to define the points of PG(n, q) as the equivalence classes of the non-zero vectors of V under the equivalence relation whereby two vectors are equivalent if one is a scalar multiple of the other.