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2. Umberto Bottazzini, “Dall’ analisi matematica al calcolo geometrico: origini delle prime ricerche di Peano,” History and Philosophy of Logic, 1985, 6, 25–52.
3. Gottlob Frege, Wissenschaftlicher Briefwechsel, ed. G. Gabriel and others ( Hamburg: Meinel, 1976 ).
4. Gottlob Frege, Nachgelassene Schriften, ed. H. Hermes and others, 2nd ed. ( Hamburg: Meinel, 1983 ).
5. Ivor Grattan-Guinness, “Living together and living apart: on the interactions between mathematics and logics from the French Revolution to the First World War,” South African journal of philosophy, 1988, 7, no. 2, 73–82.