Wilfried Sieg. In the Shadow of Incompleteness: Hilbert and Gentzen. In Peter Dybjer, Sten Lindström, Erik Palmgren, Göran Sundholm, editors, Epistemology versus Ontology - Essays on the Philosophy and Foundations of Mathematics in Honour of Per Martin-Löf. Volume 27 of Logic, Epistemology, and the Unity of Science, pages 87-127, Springer, 2012. [doi]
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