Letter to Professor Christoffer Hansteen (1826) "Oeuvres Complètes de N. H. Abel", mathematician, Nouvelle edition (1881) ed., Peter Ludwig Mejdell Sylow & Sophus Lie, Vol. 2, pp. 263-265, as quoted by Øystein Ore, "Niels Henrik Abel: Mathematician Extraordinary", p. 113, 1957.