Find all pairs (x, y) of positive integers that satisfy this equation:
(y+1)
y = 2*y
x+3y+1
*** Adapted from a problem which appeared at the 2012 Spanish Mathematical Olympiad.
If y=1 then the equation reduces to 2=6. Not a solution.
If y=2 then the equation reduces to 9=2^(x+1)+7. This would imply x=0, but that is not a positive integer.
If y=3 then the equation reduces to 64=2*3^x+10, which has a solution at x=3. (x,y)=(3,3) is a solution.
Now to show that there are no "large: solutions, y>=4.
Let y+1=w, for a bit of clarity. So the "large" solutions are w>=5.
Then the equation becomes w^(w-1) = 2*(w-1)^x + 3w-2.
Take this mod w. Then 0 = 2*(-1)^x - 2 mod w. If x is even then this is an identity. If x is odd then further reduction results in 0=4 mod w.
In the x is odd case I conclude w is a factor of 4. But these are all less than 5 so no new solutions here.
For the x is even case, go back and take the equation mod w^2.
Then 0 = 2*(1-wx) + 3w-2 mod w^2. This simplifies to 0 = (2x-3)*w mod w^2. Since w is a factor on both sides the congruence can be stepped down to 0 = 2x-3 mod w.
From here I can handwave it. For large w I can approximate the equation as w^(w-1) ~= 2*w^x. From this I expect x to be roughly the same size as x. But 2x-3 mod w implies that x is not that close, so I conclude there are no large solutions.