In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, witchwhich is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not be didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.
In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, witch is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not be didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.
In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, which is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not be didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.
In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, witch is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not be didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.
In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, witch is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.
In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, witch is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not be didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.
In Jech & Hrbacek's Introduction to Set Theory, the author adopt this notation to avoid confusion about images of sets and images of elements contained in such sets. For instance, is quite common denote $f^{-1}(\{x\}) $ by $f^{-1}(x)$; in the square brackets notation we'd write $f^{-1}[x]$, witch is more clean than $f^{-1}(\{x\})$ and not so abusive as $f^{-1}(x)$. Other reason is sets of sets: if we consider a set $A = \{A_1,\dots, A_n\}$ and a function $f:A\to B$ it would not didactic to write $f(A')$ for some $A'\subseteq A$, for the elements of $A$ is also denoted by capital letters.