Let $F(x,y)=1/sin(y-x)$
We can easily compute the right limit of $F$ on ${y=x-\pi}$ we find $-\infty$
and the left limit of $F$ on ${y=x-\pi}$ we find $+\infty$
but when we plot this function we find something weird it seems like the right and the left limit of $F$ on ${y=x-\pi}$ is $-\infty$!!!!!
can anyone explain to me this thing?