A block pressed against the vertical wall is in equilibrium. The minimum coefficient of friction is:

Here net driving force= mg-mg2=mg2downwardHence friction will act upward and its magnitude should be f = mg2If the block ‘m’ is stationary the friction between m and the wall should be static.f ≤ flimmg2≤μ.N ⇒ mg2 ≤μ(mg)  ⇒ μmin = 12