b) bP a7 → b4

a)
1.Nc5! Kb8 2.b4 a5 3.b5 a4 4.N×a4 Kb7 5.Kd7 Kb8 6.Nc5 Ka7
7.Kc6 Kb8 8.b6 Kc8 9.b7+ Kd8 10.b8=Q+ Ke7 11.Qf4 Ke8 12.Kd6 Kd8 13.Qf8#
b)
1.Kc7! b3 2.Nc5 Ka7 3.N×b3 Ka6 4.Kc6 Ka7 5.Nc5 Kb8 6.Kd7 Ka7
7.Kc7 Ka8 8.b3 Ka7 9.b4 Ka8 10.b5 Ka7 11.b6+ Ka8 12.b7+ Ka7 13.b8=Q# (Author)