# Thread: Correctness of a sample function

1. ## Correctness of a sample function

function f: {a, b, c} -> {1, 2 ,3} defined by f(a) = 1, f(b) = 1, f(c) = 2

if ~f(y) represents inverse function

is ~f({1,3}) = {a, b} correct? or should it be ~f({1}) = {a, b}

the former is printed in the book and i'm kinda confused why ~f({1, 3}) gives {a,b} since ~f({3}) gives NIL.

Appreciate any clarification

Thank you

2. Originally Posted by terencetham
function f: {a, b, c} -> {1, 2 ,3} defined by f(a) = 1, f(b) = 1, f(c) = 2

if ~f(y) represents inverse function

is ~f({1,3}) = {a, b} correct? or should it be ~f({1}) = {a, b}

the former is printed in the book and i'm kinda confused why ~f({1, 3}) gives {a,b} since ~f({3}) gives NIL.

Appreciate any clarification

Thank you
$\displaystyle f^{-1}(\{1,3\})=f^{-1}(\{1\})\cup f^{-1}(\{3\})=\{a,b\}\cup \emptyset=\{a,b\}$

where $\displaystyle \emptyset$ denotes the empty set, and so for any set $\displaystyle A$, $\displaystyle A \cup \emptyset=A$

RonL
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