bfNote. This question had a bounty, so at the end I accepted the best (and only) answer but in fact it is still open. It is (hopefully) equivalent to this question, if you have any ideas, please post them there.
bfQuestion.
Fix n. We are interested in the biggest t for which there exist two families of functions, Pi,Qi, of size t from [n] to [n] such that for any i,j whenever we consider the infinite sequence Pi(Qj(Pi(QjldotsPi(3))ldots) (where the number of iterations tends to infinity), it contains no 2's and infinitely many 1's if i=j and it contains no 1's and infinitely many 2's if inej.
bfAlowerbound.
I know a construction that shows that tge2fracn2−O(1). For every subset S of [n] that contains exactly one of 2k and 2k+1 for 2leklefracn2−2 we construct a pair of functions, PS,QS as follows. For any number m denote by m+ the smallest element of S that is bigger than m or if all elements of S are at most m then define it to be 1.
PS(1)=1,PS(2)=2 and for bigger m's PS(m)=m+, while QS(1)=1,QS(2)=2 and for bigger m's QS(m)=m if minS and QS(m)=2 if mnotinS. This way we go through all the elements of S and end in 1 if the functions have the same index, but we are pushed to 2 if they differ.
bfUpperbound.
It is of course true that tlenn. So can you do better than 2n?
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