Finite Element Analysis. Barna Szabó
of a discretization scheme, given information about the regularity (smoothness) of the exact solution and (b) a posteriori estimators that provide estimates of the error in energy norm for the finite element solution of a particular problem.
There is a very substantial body of work in the mathematical literature on the a priori estimation of the rate of convergence, given a quantitative measure of the regularity of the exact solution and a sequence of discretizations. The underlying theory is outside of the scope of this book; however, understanding the main results is important for practitioners of finite element analysis. For details we refer to [28, 45, 70, 84].
1.5.1 Regularity
Let us consider problems the exact solution of which has the functional form
where
from which it follows that α must be greater than
In the following we will see that when α is not an integer then the degree of difficulty associated with approximating
If α is a fractional number then the measure of regularity used in the mathematical literature is the maximum number of square integrable derivatives, with the notion of derivative generalized to fractional numbers. See sections A.2.3 and A.2.4 in the appendix. For our purposes it is sufficient to remember that if
If α is an integer then
Remark 1.9 The kth derivative of a function
1.5.2 A priori estimation of the rate of convergence
Analysts are called upon to choose discretization schemes for particular problems. A sound choice of discretization is based on a priori information on the regularity of the exact solution. If we know that the exact solution lies in Sobolev space
We define
(1.90)
where ℓj is the length of the jth element,
The a priori estimate of the relative error in energy norm for
(1.91)