# Goormaghtigh conjecture

In mathematics, the Goormaghtigh conjecture is a conjecture in number theory named for the Belgian mathematician René Goormaghtigh. The conjecture is that the only non-trivial integer solutions of the exponential Diophantine equation

${\displaystyle {\frac {x^{m}-1}{x-1}}={\frac {y^{n}-1}{y-1}}}$

satisfying x > y > 1 and n, m > 2 are

• (xymn) = (5, 2, 3, 5); and
• (xymn) = (90, 2, 3, 13).

This may be expressed as saying that 31 (111 in base 5, 11111 in base 2) and 8191 (111 in base 90, 1111111111111 in base 2) are the only two numbers that are repunits with at least 3 digits in two different bases.

Davenport, Lewis & Schinzel (1961) showed that, for each fixed exponents m and n, this equation has only finitely many solutions.But this proof depends on Siegel's finiteness theorem, which is ineffective. Nesterenko & Shorey (1998) showed that, if m-1=dr, n-1=ds with d ≥2, r ≥1 and s ≥1, then max (x, y, m, n) is bounded by an effectively computable constant depending only on r and s. Yuan (2005) showed that this equation has no solution (x, y, n) other than the two solutions given above for m=3 and odd n.

Balasubramanian and Shorey proved in 1980 that there are only finitely many possible solutions to the equations in (x,y,m,n) with prime divisors of x and y lying in a given finite set and that they may be effectively computed. He & Togbé (2008) showed that, for each fixed x and y, this equation has at most one solution.