Question: what are all the roman numbers?
Answer: The number of characters in the Roman numerals for 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ... (i.e., I, II, III, IV, V, VI, VII, VIII, IX, X, ...) are 1, 2, 3, 2, 1, 2, 3, 4, 2, 1, 2, 3, 4, ... (OEIS A006968).