Let G be an (additive) abelian group with subgroups H and K. Show that G H

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Let G be an (additive) abelian group with subgroups H and K. Show that G ≅ H ⊕ K  if and only if there are homomorphisms 

such that π1L1 = 1H,  π2L2 = 1K,  π1L2 = 0 and π2L1 = 0, where 0 is the map sending every element onto the zero (identity) element, and L1π1(x) + L2π2(x) = x for all x ϵ G.

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