Büchi's n-th power problem on Q asks whether there exist an integer M such that the only monic polynomials F ∈ Q[X] of degree n satisfying that F (1),. .. , F (M) are n-th power rational numbers, are precisely of the form F (X) = (X +c) n for some c ∈ Q. In this paper, we study analogues of this problem for algebraic function fields of positive characteristic. We formulate and prove an analogue (indeed, such a formulation for n > 2 was missing in the literature due to some unexpected phenomena), which we use to derive some definability and undecidability consequences. Moreover, in the case of characteristic zero we extend some known results by improving the bounds for M (from quadratic on n to linear on n). Contents Büchi's undecidability problem. Does there exists an algorithm for the following decision problem? Given diagonal quadratic forms Q j (x 1 ,. .. , x n) = a j1 x 2 1 +. .. + a jn x 2 n (for j = 1,. .. , m) with integer coefficients, and given b = (b j) j ∈ Z m , decide whether or not there is a tuple of integers a ∈ Z n such that for each j = 1,. .. , m we have Q j (a) = b j. More precisely, Büchi formulated an arithmetic problem which, if true, would imply that the previous problem has a negative answer. See [6, 8, 9] for more details. The arithmetic problem formulated by Büchi is the following.