Singular Point of An Algebraic Variety

In mathematics, a singular point of an algebraic variety V is a point P that is 'special' (so, singular), in the geometric sense that V is not locally flat there. A point of an algebraic variety which is not singular is said to be regular. An algebraic variety which has no singular point is said to be non singular or smooth.

In the case of an algebraic curve, a plane curve that has a double point, such as the cubic curve

y2 = x2(x + 1)

exhibits at (0, 0), cannot simply be parametrized near the origin. A plot of this curve is below with the singular point at the origin. An example of singular point is when a graph crosses over itself:

The reason for that algebraically is that both sides of the equation show powers higher than 1 of the variables x and y. In terms of differential calculus, if

F(x,y) = y2 − x2(x + 1),

so that the curve has equation

F(x,y) = 0,

then the partial derivatives of F with respect to both x and y vanish at (0,0). This means that if we try to use the implicit function theorem to express y as a function of x near y = 0, we shall fail; and indeed no linear combination of x and y is a function of another essentially different one, so that this is a geometric condition not tied to any choice of coordinate axes.

In general for a hypersurface

F(x, y, z, ...) = 0

the singular points are those at which all the partial derivatives simultaneously vanish. A general algebraic variety V being defined by several polynomials, or in algebraic terms an ideal of polynomials, the condition on a point P to be a singular point of V is that the linear parts of those polynomials are linearly dependent, when written in terms of variables X_i − P_i that make P the origin of coordinates.

Points of V that are not singular are called non-singular or regular. It is always true that most points are non-singular in the sense that the non-singular points form a set that is both open and non-empty.

It is important to note that the geometric criterion for a point of a variety to be singular (mentioned earlier), that it is a point where the variety is not "locally flat", can be very hard to recognize for varieties over a general field. The work of Milnor and others shows that, over the complex numbers, the statement is precisely true in every reasonable interpretation. But, as Milnor points out, over the real numbers "The equation ... can actually be solved for as a real analytic function of " (so that the variety it defines is the graph of a real analytic function, and therefore a real analytic manifold) "but this equation also defines a variety having a singular point at the origin". Obviously the "geometric" meaning of "locally flat" over fields of finite characteristic, or ultrametric fields, is even more vexed.