Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematics, specifically in the calculus of variations, the fundamental lemma in the calculus of variations is a lemma that is typically used to transform a problem from its weak formulation (variational form) into its strong formulation (differential equation). In other words, the test functions h (Ck functions vanishing at the endpoints) separate Ck functions: Ck[a,b] is a Hausdorff space in the weak topology of pairing against Ck functions that vanish at the endpoints.