We prove an asymptotic formula for the average number of solutions to the Diophantine equation axy−x−y=n in which a is fixed and n varies.
[1] Misiurewicz, M. 1966 Ungelöste Probleme Elem. Math. 21 90.
[2] Montgomery, H. L. 1971 Topics in Multiplicative Number Theory . Springer-Verlag.
[3] Montgomery, H. L., Vaughan, R. C. 2007 Multiplicative Number Theory I. Classical Theory Cambridge University Press.
[4] Titchmarsh, E. C., The Riemann Zeta-Function, 2nd ed., revised by Heath-Brown, D. R. (Oxford, 1986).
[5] Viola, C. 1973 On the diophantine equation and Acta Arith. 22 339–352.