%N Number of solutions to Pillai's equation a^x - b^y = n, with a>0, b>0, x>1, y>1.
%C This is the classic Diophantine equation of S. S. Pillai, who conjectured that there are only a finite number of solutions for each n. A generalization of Catalan's conjecture that a^x-b^y=1 has only one solution. For n <=100, a total of 274 solutions were found for perfect powers less than 10^12. No additional solutions were found for perfect powers < 10^18.
%D R. K. Guy, Unsolved Problems in Number Theory, D9.
%D T. N. Shorey and R. Tijdeman, Exponential Diophantine Equations, Cambridge University Press, 1986.
%H T. D. Noe, <a href="http://www.sspectra.com/Pillai.txt">Solutions to Pillai's Equation for n<=100</a>
%H M. A. Bennett, <a href="http://www.math.ubc.ca/~bennett/B-CJM-Pillai.pdf">On some exponential equations of S. S. Pillai</a>, Canad. J. Math. 53 (2001), 897-922.
%H Dana Mackenzie, <a href="http://math.colgate.edu/~integers/s33/s33.Abstract.html">2184: An absurd (and adsurd) tale</a>, Integers (2018) 18, Article #A33.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PillaisConjecture.html">Pillai's Conjecture</a>
%e a(4)=3 because there are 3 solutions: 4 = 2^3 - 2^2 = 6^2 - 2^5 = 5^3 - 11^2.
%Y Cf. A189117, A001597, A074981.
%A _T. D. Noe_, Oct 11 2002