General Sets
A generalized form of the diagonal argument was used by Cantor to prove Cantor's theorem: for every set S the power set of S, i.e., the set of all subsets of S (here written as P(S)), is larger than S itself. This proof proceeds as follows:
Let f be any function from S to P(S). It suffices to prove f cannot be surjective. That means that some member T of P(S), i.e., some subset of S, is not in the image of f. As a candidate consider the set:
For every s in S, either s is in T or not. If s is in T, then by definition of T, s is not in f(s), so T is not equal to f(s). On the other hand, if s is not in T, then by definition of T, s is in f(s), so again T is not equal to f(s). For a more complete account of this proof, see Cantor's theorem.
Read more about this topic: Cantor's Diagonal Argument
Famous quotes containing the words general and/or sets:
“According to the historian, they escaped as by a miracle all roving bands of Indians, and reached their homes in safety, with their trophies, for which the General Court paid them fifty pounds. The family of Hannah Dustan all assembled alive once more, except the infant whose brains were dashed out against the apple tree, and there have been many who in later time have lived to say that they have eaten of the fruit of that apple tree.”
—Henry David Thoreau (1817–1862)
“Willing sets you free: that is the true doctrine of will and freedom—thus Zarathustra instructs you.”
—Friedrich Nietzsche (1844–1900)