He had several daughters and one of them, Carola Crosio, married the famous mathematician Giuseppe Peano (of Peano axioms fame) in 1887.
In 1900, David Hilbert posed the problem of proving their consistency using only finitistic methods as the second of his twenty-three problems.
•
Gentzen, G., 1936, Die Widerspruchsfreiheit der reinen Zahlentheorie. Mathematische Annalen 112: 132–213.