more familiar to mathematicians of his time: The set of real algebraic numbers can be written as an infinite sequence in which each number appears only once. Cantor's second theorem works with a closed interval [a, b], which is the set of real numbers ≥ a and ≤ b. The theorem states: Given any sequence of real numbers x1, x2, x3, ... and any interval [a, b], there is a number in [a, b] that is not contained in the given sequence. Hence, there are infinitely many such numbers. Cantor observes that combining his two theorems yields a new proof of Liouville's theorem that every interval [a, b] contains infinitely many transcendental numbers. Cantor then remarks that his second theorem is: This remark contains Cantor's uncountability theorem, which only states that an interval [a, b] cannot be put into one-to-one correspondence with the set of positive integers. It does not state that this interval is an infinite set of larger cardinality than the set of positive integers. Cardinality is defined in Cantor's next article, which was published in 1878. Cantor only states his uncountability theorem. He does not use it in any proofs. The proofs First theorem To prove that the set of real algebraic numbers is countable, define the height of a polynomial of degree n with integer coefficients as: n − 1 + |a0| + |a1| + ... + |an|, where a0, a1, ..., an are the coefficients of the polynomial. Order the polynomials by their height, and order the real roots of polynomials of the same height by numeric order. Since there are only a finite number of roots of polynomials of a given height, these orderings put the real algebraic numbers into a sequence. Cantor went a step further and produced a sequence in which each real algebraic number appears just once. He did this by only using polynomials that are irreducible over the integers. The following table contains the beginning of Cantor's enumeration. Second theorem Only the first part of Cantor's second theorem needs to be proved. It states: Given any sequence of real numbers x1, x2, x3, ... and any interval [a, b], there is a number in [a, b] that is not contained in the given sequence. To find a number in [a, b] that is not contained in the given sequence, construct two sequences of real numbers as follows: Find the first