About the representation of solutions of linear elliptic equations of arbitrary order Wildenhain, Günther — 1982 Equadiff 5
Über einen Kapazitätsbegriff bei der Differentialgleichung ...u = 0. Günther Wildenhain — 1967 Mathematische Zeitschrift
Approximation by solutions of general boundary value problems for elliptic equations of arbitrary order Uwe Hamann; Günther Wildenhain — 1987 Banach Center Publications