sequoia13

Thank you!

Does anyone know where I can find an elementary proof (not using homology...) that there is no retraction from $D^n$ to $S^{n-1}$?

If $F:X\times I\to Y$ is a homotopy s.t. $x_1\sim x_2\implies f(x_1,t)=f(x_2,t)\forall t\in I$, how to show that $G:(X/\sim)\times I\to Y$ defined so $([x],t)\mapsto f(x,t)$ is continuous?

Hello. If X is the space of rationals with usual topology. How to calculate the fundamental group $\pi_1(X,100)$?