Geometric progression modulo p
Ok, I'll post a new question. The first question was not really what I needed, it would just easily (but wrong) imply what I need. Well, to discover $s$ is an annoying handwork (I cannot programming), so I confess I didn't check many cases ($p=11,n=5$ only). The second question also implies what I need, but not too easy. I checked some cases by hand ($(n,p,k,s) = (5,11,6,4), (5,31,\{6,11,16,21,26\},16), (7,29,\{8,15,22\},\{7,16\})$) and it really does not seem to be false. An obvious observation is that $2 \le s \le \min\{k-1,q-k\}$.