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Aug 17, 2017 at 19:05 comment added user43326 @AliTaghavi if $n<m$, then the natural inclusion has the property stated in the question.
Aug 16, 2017 at 17:56 comment added Ali Taghavi @NeilStrickland(and @Tsemo)) what can be said about the homotopy class of any such map when n=m). For n=m=1, the map can not be null homotopic but what about higher values of n=m). What can think to the same question for n<m.
Aug 16, 2017 at 17:36 vote accept Ali Taghavi
Aug 16, 2017 at 17:36 comment added Ali Taghavi @NeilStrickland (and @Tsemo) My apology for my bad and trivial question. it is fair that the question will be voted to closed. thanks for help of you.
Aug 16, 2017 at 14:42 comment added Neil Strickland Or more directly: just choose $n+1$ mutually orthogonal lines in $\mathbb{C}^{n+1}$, and apply $f$ to obtain $n+1$ mutually orthogonal lines in $\mathbb{C}^{m+1}$, which is impossible.
Aug 16, 2017 at 12:11 comment added Tsemo Aristide It is the image of $f$ restricted to the projective hyperplane $[H_x]$.
Aug 16, 2017 at 12:08 comment added Ali Taghavi So what does it mean $f([H_x])$? f is defined on the projective space not on grassmanian.
Aug 16, 2017 at 12:05 comment added Tsemo Aristide It is the orthogonal hyperplane to $x$. I fix an $x$, no need to choose $H_x$ continuously.
Aug 16, 2017 at 11:35 comment added Ali Taghavi What is $H_x$? is it a line?How do you choose it, continuously?
Aug 16, 2017 at 11:27 history answered Tsemo Aristide CC BY-SA 3.0