1. Abraham, R.: Foundations of Mechanics. W.A. Benjamin, New York 1967.
2. Anon: Christoffel, E. B. Boll. di bibliografia e storia d. sci. mat., Turin/Geneva (= Loria) 4 (1901), 57–58.
3. Atkinson, F.V.: Christoffel’s work on shock waves. This Christoffel-Festschrift.
4. Azzam, A., and Kreyszig, E.: Regularity properties of solutions of elliptic equations near corners. This Christoffel-Festschrift.
5. Bacharach, M.: Abriss der Geschichte der Potentialtheorie. Vandenhoek und Ruprecht, Göttingen 1883 (see especially pp. 13–15, 22, 42, 55, 66, 71, 74 ).