Thanks, Jinjing. I am trying to generalize your method since you mensioned 9781 is a prime. Then let's take a look at another equation: x^2 - 83*y^2 = 1. 83 is a prime.
Let y = 3^2, then (x+1)(x-1)= 83*3^4=83*81, (x+1)-(x-1) = (83*3^0 - 3^4)*3^0 = 2. Then we have x+1=83 and x-1=81, so (82, 9) is an intergral root. Notice that 3^0, that is the issue.
You method is more promising if you can amend a little bit. Even computer cannot solve number theory problems because of the storage capacity limitation. I suspect the following equations have no integral soltions either.
x^2 - 61*y^2 = 1
x^2 - 89*y^2 = 1
x^2 - 97*y^2 = 1