像 (数学)
f → : P ( X ) → P ( Y ) ; A ↦ f → ( A ) = { f ( a ) ∣ a ∈ A } . {\displaystyle f^{\to }\colon {\mathfrak {P}}(X)\to {\mathfrak {P}}(Y);\;A\mapsto f^{\to }(A)=\{f(a)\mid a\in A\}.}
f ← : P ( Y ) → P ( X ) ; B ↦ f ← ( B ) = { a ∈ X ∣ f ( a ) ∈ B } . {\displaystyle f^{\gets }\colon {\mathfrak {P}}(Y)\to {\mathfrak {P}}(X);\;B\mapsto f^{\gets }(B)=\{a\in X\mid f(a)\in B\}.}