Eternal non-Markovianity of qubit maps

As is well known, unital Pauli maps can be eternally non-CP-divisible. In contrast, here we show that in the case of nonunital maps, eternal non-Markovianity in the nonunital part is ruled out. In the unital case, the eternal non-Markovianity can be obtained by a convex combination of two dephasing semigroups, but not all three of them. We study these results and ramifications arising from them.