= Linear and second-order response of the differential pom-pom model
{title2=$G_{\rm rel}(t)=\frac G3e^{-t/\tau_1},\quad\mu_0=\frac{G\tau_1}{3},\quad\psi_1=\frac{2G\tau_1^2}{3},\quad\psi_2=0$}
Linearizing the <differential pom-pom model> about its relaxed state leaves no first-order stretch perturbation in <incompressible flow>. Its anisotropic orientation obeys $\dot a+a/\tau_1=A_1$, producing the displayed <relaxation modulus>. In a slow-flow expansion, $A=I+\tau_1A_1+\tau_1^2(2A_1^2-A_2)+O(3)$. Trace normalization and the first stretch correction affect only isotropic <stress> at this order, giving the displayed <second-order fluid> coefficients. Linear small-amplitude response allows arbitrary frequency, whereas this local second-order expansion also requires slow time dependence.
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