I1m studying fundamental group and its relation with covering maps, I was thinking about an exercise: every isomorphism of $\pi_{1}(T^2, x_{0})$ with itself is induced by a homomorphism $f:T^2 \rightarrow T^2$ . $T^2 = S^1 \times S^1.$
I probably have to use a matrix with integer entries and induce a continus function, but I don't know hou to that.