Today my fellow grad student asked me a question, given a map f from X to Y, assume $f_*(\pi_i(X))=0$ in Y, when is f null-homotopic?

I search the literature a little bit, D.W.Kahn 

[Link](https://projecteuclid.org/journals/pacific-journal-of-mathematics/volume-15/issue-2/Maps-which-induce-the-zero-map-on-homotopy/pjm/1102995805.full)

And M.Sternstein has worked on this, and Sternstein even got a necessary and sufficient condition, for suitable spaces. 

http://www.jstor.org/stable/pdfplus/2037939.pdf

However, his condition is a little complicated for me as a beginner. Right now I just wanted a counter example of a such a map. Kahn in his paper said one can have many such examples using Eilenberg Maclance spaces. Well, we can certainly show a lot of map between E-M spaces induce zero map on homopoty groups just by pure group theoretic reasons, but I can not think of a easy example when you can show that map, if it exists, is not null-homotopic. Could someone give me some hint?

or, maybe even some examples arising from manifolds?