Richard Zach
with Schiemer, G. , Reck, E. (2017). Carnap's early metatheory: scope and limits. Synthese, 194 (1), 33-65. https://doi.org/10.1007/s11229-015-0877-z.
(2003). The practice of finitism: epsilon calculus and consistency proofs in Hilbert's program. Synthese, 137 (1-2), 211-259. https://doi.org/10.1023/A:1026247421383.