Since (x−1)⋅x⋅(x+1)+(y−1)⋅y⋅(y+1)=x3+y3−x−y, adding 3xy(x+y) to both sides of the equation yields the equivalent equation
(x+y)3−(x+y)=24+3xy(x+y−3)⟺(x+y)3−27−(x+y−3)=3xy(x+y−3).
Since (x+y)3−27=(x+y−3)((x+y)2+3(x+y)+9), this is equivalent to
(x+y−3)((x+y)2+3(x+y)+9−1−3xy)=0⟺(x+y−3)(x2−xy+y2+3x+3y+8)=0.
If the expression x+y−3 is equal to 0, we obtain the set of solutions
{(t,3−t); t∈Z}.
It remains to find all solutions of the equation x2−xy+y2+3x+3y+8=0.
If we consider the equivalent equation x2−(y−3)⋅x+y2+3y+8=0 as a quadratic equation in x, the discriminant (y−3)2−4(y2+3y+8)=−3y2−18y−23=4−3(y+3)2
must be a perfect square if the solutions are to be integers. This is the case iff (y+3)2=0∨(y+3)2=1, i.e. iff y=−2∨y=−3∨y=−4.
For y=−2 we obtain the equation x2+5x+6=0 for x, and thus the solutions (−2,−2) and (−3,−2).
For y=−3 we obtain the equation x2+6x+8=0 for x, and thus the solutions (−2,−3) and (−4,−3).
Finally, for y=−4 we obtain the equation x2+7x+12=0 for x, and thus the solutions (−3,−4) and (−4,−4), completing the set of solutions. □