Unisolvent functions


In mathematics, a set of n functions f1, f2,..., fn is unisolvent on a domain Ω if the vectors
are linearly independent for any choice of n distinct points x1, x2... xn in Ω. Equivalently, the collection is unisolvent if the matrix F with entries fi has a nonzero determinant: det ≠ 0 for any choice of distinct xj's in Ω. Unisolvency is a property of vector spaces, not just particular sets of functions. That is, a vector space of functions of dimension n is unisolvent if given any basis, the basis is unisolvent. This is because any two bases are related by an invertible matrix, so one basis is unisolvent if and only if any other basis is unisolvent.
Unisolvent systems of functions are widely used in interpolation since they guarantee a unique solution to the interpolation problem. The set of polynomials of degree at most are unisolvent by the unisolvence theorem.

Examples

  • 1, x, x2 is unisolvent on any interval by the unisolvence theorem
  • 1, x2 is unisolvent on, but not unisolvent on
  • 1, cos, cos,..., cos, sin, sin,..., sin is unisolvent on
  • Unisolvent functions are used in linear inverse problems.

Unisolvence in the finite element method

When using "simple" functions to approximate an unknown function, such as in the finite element method, it is useful to consider a set of functionals that act on a finite dimensional vector space of functions, usually polynomials. Often, the functionals are given by evaluation at points in Euclidean space or some subset of it.
For example, let be the space of univariate polynomials of degree or less, and let for be defined by evaluation at equidistant points on the unit interval. In this context, the unisolvence of with respect to means that is a basis for, the dual space of. Equivalently, and perhaps more intuitively, unisolvence here means that given any set of values, there exists a unique polynomial such that. Results of this type are widely applied in polynomial interpolation; given any function on, by letting, we can find a polynomial that interpolates at each of the points:.

Dimensions

Systems of unisolvent functions are much more common in 1 dimension than in higher dimensions. In dimension d = 2 and higher, the functions f1, f2,..., fn cannot be unisolvent on Ω if there exists a single open set on which they are all continuous. To see this, consider moving points x1 and x2 along continuous paths in the open set until they have switched positions, such that x1 and x2 never intersect each other or any of the other xi. The determinant of the resulting system is the negative of the determinant of the initial system. Since the functions fi are continuous, the intermediate value theorem implies that some intermediate configuration has determinant zero, hence the functions cannot be unisolvent.