Lemma 21.39.9. Let $\mathcal{C}_ i$, $i = 1, 2$ be categories. Let $u_ i : \mathcal{C}_1 \times \mathcal{C}_2 \to \mathcal{C}_ i$ be the projection functors. Let $B$ be a ring. Let $g_ i : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}_1 \times \mathcal{C}_2), \underline{B}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}_ i), \underline{B})$ be the corresponding morphisms of ringed topoi, see Example 21.39.3. For $K_ i \in D(\mathcal{C}_ i, B)$ we have

\[ L(\pi _1 \times \pi _2)_!( g_1^{-1}K_1 \otimes _{\underline{B}}^\mathbf {L} g_2^{-1}K_2) = L\pi _{1, !}(K_1) \otimes _ B^\mathbf {L} L\pi _{2, !}(K_2) \]

in $D(B)$ with obvious notation.

**Proof.**
As both sides commute with colimits, it suffices to prove this for $K_1 = j_{U!}\underline{B}_ U$ and $K_2 = j_{V!}\underline{B}_ V$ for $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}_1)$ and $V \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}_2)$. See construction of $L\pi _!$ in Lemma 21.37.2. In this case

\[ g_1^{-1}K_1 \otimes _{\underline{B}}^\mathbf {L} g_2^{-1}K_2 = g_1^{-1}K_1 \otimes _{\underline{B}} g_2^{-1}K_2 = j_{(U, V)!}\underline{B}_{(U, V)} \]

Verification omitted. Hence the result follows as both the left and the right hand side of the formula of the lemma evaluate to $B$, see construction of $L\pi _!$ in Lemma 21.37.2.
$\square$

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