26. Find two integers such that their product equals the sum and difference of these numbers.
26. Let the required numbers be $a$ and $b$. Then, by the condition, $a+b=ab$, from which $a=b:(b-1)$. This number is an integer only when $b=a=2$ or $a=b=0$. According to the condition, $ab=a-b$ as well, from which $b=a:(a+1)$, which gives an integer when $a=b=0$ or $b=2$ and $a=-2$. But in the second case, $a+b=0$, so it does not satisfy the condition. Therefore, $ab=a+b=a-b$ when $a=b=0$.