# A Simple Introduction to the Mixed Finite Element Method: by Gabriel N. Gatica

By Gabriel N. Gatica

The major objective of this booklet is to supply an easy and available creation to the combined finite point approach as a primary software to numerically resolve a large type of boundary worth difficulties coming up in physics and engineering sciences. The booklet relies on fabric that was once taught in corresponding undergraduate and graduate classes on the Universidad de Concepcion, Concepcion, Chile, over the last 7 years. compared with numerous different classical books within the topic, the most good points of the current one need to do, on one hand, with an test of featuring and explaining many of the info within the proofs and within the varied purposes. particularly a number of effects and facets of the corresponding research which are frequently on hand in basic terms in papers or lawsuits are incorporated here.

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Extra info for A Simple Introduction to the Mixed Finite Element Method: Theory and Applications

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3. 43) but to an equivalent formulation. To this end, it is necessary to previously notice some properties of the bilinear forms a and b. 4 Application Examples 43 1 where, given τ ∈ L2 (Ω ), τ d := τ − tr(τ ) I is the corresponding deviator tensor. 48) will become clear later on when we prove that a is elliptic in the kernel of the operator induced by b. In turn, it is easy to see that b(I, (v, η )) = 0 ∀ (v, η ) ∈ Q. 49) We now notice that H can be decomposed as H := H0 ⊕ R I, where H0 := τ ∈ H(div; Ω ) : Ω tr(τ ) = 0 .

4 Application Examples 49 Proof. Let τ := τ 0 + d I ∈ HΓN (div; Ω ), with τ 0 ∈ H0 and d ∈ R. 27), we deduce that |d| n −1/2,00,ΓN = γ n (τ 0 )|ΓN −1/2,00,ΓN ≤ γ n (τ 0 ) −1/2,Γ ≤ τ0 div,Ω , that is, |d| ≤ 1 n −1/2,00,ΓN τ0 div,Ω . The preceding inequality, together with the fact that τ 2 div,Ω = τ0 2 div,Ω + n d 2 |Ω | , implies that τ 2 div,Ω ≤ 1+ n|Ω | n 2 −1/2,00,ΓN τ0 2 div,Ω , which completes the proof. We now observe that B : H → Q, the operator induced by the present bilinear form b : H × Q → R [cf.

Furthermore, it is important to remark that a sufficient (but not necessary) condition for (i), which appears very often in applications, is the V -ellipticity of the bilinear form a, which means (cf. 3) that there exists α > 0 such that a(τ , τ ) ≥ α τ ∀τ ∈ V . 1, is the following. 3. Let V := N(B) and assume that: (i) The bilinear form a is V -elliptic [cf. 15)]. 7)]. 5)]. Moreover, there exists a constant C > 0, which depends on A , α , and β , such that (σ , u) H×Q ≤C F H + G Q . Proof. 1. , [13, 26, 31, 45]).