20 Yann Bugeaud
Their method rests on various more or less classical techniques in Diophantine approx-
imation, including linear forms in p-adic logarithms [1], together with some heavy
computations.
The aim of the present note is to explain how Inkeri's result can be extended to
other values of the basis x, and to solve (1) completely for χ < 100 and χ = 1000.
2. Statement of the results
We obtain the following extension of Theorem 7 of Inkeri [5].
Theorem. The only solutions of equation (1) with χ < 100 or χ = 1000 are given by
(α, χ, y, n, q) e {(1, 3, 11, 5, 2), (1, 7, 20,4, 2), (4, 7,40,4, 2),
(1,18,7, 3, 3), (7,18,49, 3,2), (7,18,7, 3,4),
(8,18,14, 3, 3), (3, 22, 39, 3,2), (12,22,78, 3,2),
(19, 30, 133, 3, 2), (21,41,1218,4, 2), (13, 68, 247, 3, 2),
(52,68,494, 3,2), (58,99, 7540,4,2)}.
Remarks. All these solutions have been already found by Inkeri, but he did not show
that these are the only ones. Our proof depends considerably on the case a = 1 in
which (1) is already solved. As pointed out by Inkeri [5], "this reveals the decisive
importance of the case a = 1 in the study of the Diophantine equation (1)".
From the results of [4], [3] and of the present note, it turns out that we have at our
disposal an algorithm which enables one to solve (1) for any fixed given value of * ...
provided, of course, that χ is not too large, otherwise the computation would take too
much time.
Our theorem includes the fact that, if a, b and c denote any digits 0,..., 9, then
none of the non-zero numbers
αα.,.αα, abab...abab and abcabc...abcabc
written in basis ten is a perfect power, except, of course, the numbers a, ab and abc
when these are perfect powers.
3. Auxiliary results
In the sequel of this note, we use the following notation. For a prime number ρ and
an integer l non divisible by p, we denote by οτάρ(ί) the least positive integer m such
that im = 1 (mod p).