Per the Master Theorem, the three cases for T(n) = a · T(n/b) + c n^d are decided by comparing

Master theorem compares a (sub-problem count) to b^d (per-level divide-work growth) to decide which level dominates. a < b^d: top dominates, O(n^d). a = b^d: all levels equal, O(n^d log n). a > b^d: bottom dominates, O(n^log_b a). The other expressions are nonsense for this purpose.