In '70 Freyd proved that the homotopy category of spaces is not concrete. We significantly
generalize this result to a vast class of homotopy category of model categories. We outline
further questions related to the subject.
Links:
[1] https://www.mpim-bonn.mpg.de/de/taxonomy/term/39
[2] https://www.mpim-bonn.mpg.de/de/node/3444
[3] https://www.mpim-bonn.mpg.de/de/node/3207