-http://en.wikipedia.org/wiki/Real_projective_plane
-http://en.wikipedia.org/wiki/Roman_surface
-http://en.wikipedia.org/wiki/Cross_cap
-http://en.wikipedia.org/wiki/Moebius_strip
+https://en.wikipedia.org/wiki/Real_projective_plane
+https://en.wikipedia.org/wiki/Roman_surface
+https://en.wikipedia.org/wiki/Cross_cap
+https://en.wikipedia.org/wiki/Moebius_strip