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The achievement of Lax and Milgram in their 1954 result was to specify sufficient conditions for this weak formulation to have a unique solution that depends continuously upon the specified datum f ∈ V ∗: it suffices that U = V is a Hilbert space, that B is continuous, and that B is strongly coercive, i.e.
Ivo M. Babuška (22 March 1926 – 12 April 2023) was a Czech-American mathematician, noted for his studies of the finite element method and the proof of the Babuška–Lax–Milgram theorem in partial differential equations. [1]
This is a formulation of the Lax–Milgram theorem which relies on properties of the symmetric part of the bilinear form. It is not the most general form. It is not the most general form. Let V {\displaystyle V} be a real Hilbert space and a ( ⋅ , ⋅ ) {\displaystyle a(\cdot ,\cdot )} a bilinear form on V {\displaystyle V} , which is
The first question — the shape of the domain — is the one in which the power of the Lions–Lax–Milgram theorem can be seen. In simple settings, it suffices to consider cylindrical domains : i.e., one fixes a spatial region of interest, Ω, and a maximal time, T ∈(0, +∞], and proceeds to solve the heat equation on the "cylinder"
One may show, via the Lax–Milgram lemma, that whenever (,) is coercive and () is continuous, then there exists a unique solution () to the weak problem (*). If further A ( u , φ ) {\displaystyle A(u,\varphi )} is symmetric (i.e., b = 0 {\displaystyle b=0} ), one can show the same result using the Riesz representation theorem instead.
Created Date: 8/30/2012 4:52:52 PM
By the Lax-Milgram theorem (see weak formulation), these two conditions imply well-posedness of the original problem in weak formulation. All norms in the following sections will be norms for which the above inequalities hold (these norms are often called an energy norm).
The existence theory for elliptic operators, using Gårding's inequality, Lax–Milgram lemma and Fredholm alternative, states the sufficient condition for a weak solution u to exist in the Sobolev space H k. For example, for a Second-order Elliptic operator as in Example 2,