Lemma 10.5.2. Let R be a ring. Let \alpha : R^{\oplus n} \to M and \beta : N \to M be module maps. If \mathop{\mathrm{Im}}(\alpha ) \subset \mathop{\mathrm{Im}}(\beta ), then there exists an R-module map \gamma : R^{\oplus n} \to N such that \alpha = \beta \circ \gamma .
Proof. Let e_ i = (0, \ldots , 0, 1, 0, \ldots , 0) be the ith basis vector of R^{\oplus n}. Let x_ i \in N be an element with \alpha (e_ i) = \beta (x_ i) which exists by assumption. Set \gamma (a_1, \ldots , a_ n) = \sum a_ i x_ i. By construction \alpha = \beta \circ \gamma . \square
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