In a commutative diagram of two exact rows A₁→A₂→A₃→A₄→A₅ and B₁→B₂→B₃→B₄→B₅, if the outer vertical maps are suitably iso/epi/mono, the middle vertical is an isomorphism. The workhorse of diagram-chasing in abelian categories.
In a commutative diagram of two exact rows A₁→A₂→A₃→A₄→A₅ and B₁→B₂→B₃→B₄→B₅, if the outer vertical maps are suitably iso/epi/mono, the middle vertical is an isomorphism. The workhorse of diagram-chasing in abelian categories.