Spectral properties of the gradient operator with nonconstant coefficients

In mathematical physics, the gradient operator with nonconstant coefficients encompasses various models, including Fourier’s law for heat propagation and Fick’s first law, that relates the diffusive flux to the gradient of the concentration. Specifically, consider n≥ 3 n ≥ 3 orthogonal unit vectors e₁,… ,eₙ∈ Rⁿ e 1 , … , e n ∈ R n , and let Ω ⊆ Rⁿ Ω ⊆ R n be some (in general unbounded) Lipschitz domain. This paper investigates the spectral properties of the gradient operator T=∑ ᵢ₌₁ⁿeᵢaᵢ(x)∂ /∂ xᵢ T = ∑ i = 1 n e i a i ( x ) ∂ ∂ x i with nonconstant positive coefficients aᵢ:Ω ̄→ (0,∞ ) a i : Ω ¯ → ( 0 , ∞ ) . Under certain regularity and growth conditions on the aᵢ a i , we identify bisectorial or strip-type regions that belong to the S -resolvent set of T . Moreover, we obtain suitable estimates of the associated resolvent operator. Our focus lies in the spectral theory on the S -spectrum, designed to study the operators acting in Clifford modules V over the Clifford algebra Rₙ R n , with vector operators being a specific crucial subclass. The spectral properties related to the S -spectrum of T are linked to the inversion of the operator Qₛ(T):=T²-2s₀T+|s|² Q s ( T ) : = T 2 - 2 s 0 T + | s | 2 , where s∈ Rⁿ⁺¹ s ∈ R n + 1 is a paravector, i.e., it is of the form s=s₀+s₁e₁+⋯ +sₙeₙ s = s 0 + s 1 e 1 + ⋯ + s n e n . This spectral problem is substantially different from the complex one, since it allows to associate general boundary conditions to Qₛ(T) Q s ( T ) , i.e., to the squared operator T² T 2 .