8. Find all integer values for $X$ and $Y$ such that $X^{4}+4 Y^{4}$ is a prime number.
64.8. Since $X^{4}+4 Y^{4}=\left(X^{2}+2 X Y+2 Y^{2}\right) \quad\left(X^{2}-2 X Y+\right.$ $+2 Y^{2}$ ), this number is prime only when $X=Y=1$ or $X=Y=-1$.