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On the Diophantine equation 1k + 2k + ⋯ + xk = yn

2004, Compositio Mathematica

Abstract

In this paper, we resolve a conjecture of Schäffer on the solvability of Diophantine equations of the shape 1 k + 2 k + · · · + x k = y n , for 1 k 11. Our method, which may, with a modicum of effort, be extended to higher values of k, combines a wide variety of techniques, classical and modern, in Diophantine analysis.

Key takeaways

  • One of the main interests in this result is that the proof requires a combination of virtually every technique in modern Diophantine analysis, including local methods, a classical reciprocity theorem in cyclotomic fields, lower bounds for linear forms in logarithms of algebraic numbers and in elliptic logarithms, a computational method for finding the solutions to elliptic equations, the hypergeometric method of Thue and Siegel, and results on ternary equations based upon Galois representations and modular forms.
  • For example, if we suppose that a n − Db n = 1 with n coprime to φ(D) and |a| > 1, then, if p divides D, a n ≡ 1 (mod p) and so a ≡ 1 (mod p).
  • As noted previously, equations with |A − B| = 1 have, via Lemma 4.4, only solutions (a, b) with |ab| = 1.
  • x, x + 1 ∈ {a n , 2 × 3 δ 3 5 (n−1)/2 b n }, δ 3 ∈ {0, (n + 1)/2} and so, if δ 3 = 0, T 11 (x) = 30t n for some positive integer t. Since (5 (n−1)/2 ) n−1 ≡ 1 (mod n 2 ) implies that 5 n−1 ≡ 1 (mod n 2 ), Corollary 6.2 and Lemma 6.3 yield n 2 |x(x + 1).
  • Let us suppose we have a solution in non-zero integers a and b to Equation (28) (so that |ab| > 1), with n 7 prime, and consider the elliptic curve E : y 2 + 3xy − Db n y = x 3 , where D = 2 × 3 (n+1)/2 × 5 (n−1)/2 .