dtgreene: Here is my proof that all positive integers are equal. Specifically, I show that, given every subset of the integers, every integer in that subset is equal. Your task is to find a mistake in this proof, which uses the technique of mathematical induction.
First, the base cast: given a set of 1 integer, it is obvious that every integer in the set is equal, as there are no two unequal integers in the set (because there are no two integers in the set in the first place).
Now, suppose that, for every set of n integers, every integer in the set is equal. We now take a set of n+1 integers. The first through nth integers in the set are equal because they form a set of n integers. The same can be said of the second through (n+1)st integers in the set. Hence, the first element of the set must equal every member of the intersection of those two subsets, and the same can be said of the (n+1)st element. Therefore, every integer in the set is equal.
Now, your task, as I mentioned above, is to find a mistake in this proof.
The inductive step doesn't work when n+1=2 since the two subsets you create have nothing in common.