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Mathematics 21 Online
OpenStudy (idku):

Just a definition-question; (about powerset and a set)

OpenStudy (idku):

So, suppose I have a set \(\color{black}{\small \rm\displaystyle B}\), and a power-set of this set \(\color{black}{\small \rm\displaystyle B}\), denoted as \(\color{black}{\small \rm \displaystyle P(B)}\). Suppose also that \(\color{black}{ \displaystyle x}\) is an arbitrary element in \(\color{black}{ \displaystyle B}\). Is it true then, that \(-\) if \(\color{black}{ \displaystyle x\in B}\) then \(\color{black}{ \displaystyle x\in P(B)}\)?

ganeshie8 (ganeshie8):

Short answer is no. Do you see any difference between \(x\) and \(\{x\}\) ?

OpenStudy (idku):

I think that \({x}\) is an element, and \(\{x\}\) is a set.

ganeshie8 (ganeshie8):

Yes, what's the definition of a powerset ?

OpenStudy (idku):

The set of all possible subsets of the set.

ganeshie8 (ganeshie8):

Notice that a subset is a set.

OpenStudy (idku):

Yes, I know, and that what I was asking \(-\) whether or not \(x\) can also be called a set or not.

OpenStudy (idku):

because, if not, the statement is false, and if yes, the statement is true.

ganeshie8 (ganeshie8):

\(x\) and \(\{x\}\) are two different things. \(x\) is an element, \(\{x\}\) is a set with one element \(x\).

OpenStudy (idku):

So, \(x\) is definitely \(\color{red}{\bf not}\) a set?

OpenStudy (idku):

(all I want to verify)

ganeshie8 (ganeshie8):

Yes. \(x\not \in P(B)\) The elements of the powerset of B are subsets of B, not elements of B. \(\{x\} \in P(B)\)

OpenStudy (idku):

I see, so if I make either of the two statements below, the they are false? \(*\) If \(\color{black}{ \displaystyle x\in B}\), then \(\color{black}{ \displaystyle x\in P(B)}\). \(*\) If \(\color{black}{ \displaystyle x\in P(B)}\), then \(\color{black}{ \displaystyle x\in B}\).

ganeshie8 (ganeshie8):

Both false

OpenStudy (idku):

Yes, thank you for confirming! (Now I am ready to go with my proof) Have a nice weekend!

ganeshie8 (ganeshie8):

Np, you too have a nice weekend :)

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