Basic Category Theory
《Basic Category Theory》 · 第 94 页 · Theorem 4.2.1
Let \(\mathcal A\) be locally small. For \(A\in\mathcal A\) and \(X:\mathcal A^{\mathrm{op}}\to\mathbf{Set}\), there is a bijection \([\mathcal A^{\mathrm{op}},\mathbf{Set}](H_A,X)\cong X(A)\), natural in \(A\) and \(X\).