Without Loss of Generality in Differential Equations

1. Suppose we want to prove a theorem [Zyg, vol.1, p.59, Lemma 8.11] for the general case. In order to restrict our consideration to a standard case without loss of generality, we invent a device X which allows us to transform the general case to a standard case.
   Example. Suppose fÎL¹.
       By [Zyg, vol.1, p.51, l.-2-l.-1], S^~[f] at x₀ is the same thing as S^~[y] at t = 0, where y(t) = [f(x₀+t)-f(x₀-t)]/2.
   Thus, X(f) = y is the device that allows us to assume that x₀ = 0 and that f is odd without loss of generality. One may wonder whether the device X can produce a new element if x₀ = 0 and f is odd. The answer is no: X(f) = f [Zyg, vol.1, p.59, l.-9]. Consequently, X(X(f)) = X(f).
       Before we prove the theorem for the standard case, we should verify that X(f) satisfies the hypothesis of the theorem just like f does. After we prove the theorem for the standard case, we should be able to derive the conclusion of the general case from the conclusion of the standard case.