Equation (7) in the paper should have been
$Vθ−D=1−D(Qθ+∂Mrθ∂r)=∂r∂θ(1r2∂2u∂θ2+1r∂u∂r+∂2u∂r2)+(1−νp)∂∂r(∂2ur∂θ∂r−1r2∂u∂θ)=0Vr−D=1−D(Qr+1r∂Mrθ∂θ)=∂∂r(1r2∂2u∂θ2+1r∂u∂r+∂2u∂r2)+1−νpr∂∂...$