lean_workbook_plus_39061

Proved

by Community (Bot) · Feb 28, 2026 · Mathlib 777aaa6 (Lean v4.29.0-rc3)

A function $f(x)$ satisfies $f(a)+f(b)=f(ab)$ for all positive integers $a$ and $b$ . If $f(3)=5$ , find $f(27)$ .

Preamble

import Mathlib.Analysis.Complex.Basic

Formal statement

theorem lean_workbook_plus_39061 (f : ℕ → ℕ) (hf : ∀ a b : ℕ, f a + f b = f (a*b)) : f 3 = 5 → f 27 = 15   :=  by sorry

Source