Lemma 13.9.2. Suppose that

\[ \xymatrix{ K_1^\bullet \ar[r]_{f_1} \ar[d]_ a & L_1^\bullet \ar[d]^ b \\ K_2^\bullet \ar[r]^{f_2} & L_2^\bullet } \]

is a diagram of morphisms of complexes which is commutative up to homotopy. Then there exists a morphism $c : C(f_1)^\bullet \to C(f_2)^\bullet $ which gives rise to a morphism of triangles $(a, b, c) : (K_1^\bullet , L_1^\bullet , C(f_1)^\bullet , f_1, i_1, p_1) \to (K_2^\bullet , L_2^\bullet , C(f_2)^\bullet , f_2, i_2, p_2)$ of $K(\mathcal{A})$.

**Proof.**
Let $h^ n : K_1^ n \to L_2^{n - 1}$ be a family of morphisms such that $b \circ f_1 - f_2 \circ a= d \circ h + h \circ d$. Define $c^ n$ by the matrix

\[ c^ n = \left( \begin{matrix} b^ n
& h^{n + 1}
\\ 0
& a^{n + 1}
\end{matrix} \right) : L_1^ n \oplus K_1^{n + 1} \to L_2^ n \oplus K_2^{n + 1} \]

A matrix computation show that $c$ is a morphism of complexes. It is trivial that $c \circ i_1 = i_2 \circ b$, and it is trivial also to check that $p_2 \circ c = a \circ p_1$.
$\square$

## Comments (3)

Comment #290 by arp on

Comment #1554 by jpg on

Comment #1572 by Johan on