# Hilbert's seventh problem

Hilbert's seventh problem is one of David Hilbert's list of open mathematical problems posed in 1900. It concerns the irrationality and transcendence of certain numbers (Irrationalität und Transzendenz bestimmter Zahlen).

## Statement of the problem

Two specific questions are asked:

1. In an isosceles triangle, if the ratio of the base angle to the angle at the vertex is algebraic but not rational, is then the ratio between base and side always transcendental?
2. Is $a^b$ always transcendental, for algebraic $a \not\in \{0,1\}$ and irrational algebraic $b$?

## Solution

The second question was answered in the affirmative by Aleksandr Gelfond in 1934, and refined by Theodor Schneider in 1935. This result is known as Gelfond's theorem or the Gelfond–Schneider theorem. (The restriction to irrational b is important, since it is easy to see that $a^b$ is algebraic for algebraic a and rational b.)

From the point of view of generalisations, this is the case

$b \ln{\alpha} + \ln{\beta} = 0$

of the general linear form in logarithms which was attacked by Gelfond and then solved by Alan Baker. It is called the Gelfond conjecture or Baker's theorem. Baker was rewarded as a Fields Medal winner in 1970 because of this.

The first question is a consequence of the second question.