Question

On the Wikipedia page for the Axiom of Choice the following statement is given:

$(\forall x^\sigma)(\exists y^\tau)R(x,y)\rightarrow(\exists f^{\sigma \rightarrow \tau})(\forall x^\sigma)R(x, f(x))$

Most of it seems fairly straightforward, except for the meanings of the symbols that look like 180 degree rotated 'E' and 'A'

Was it helpful?

Solution

The symbols are quantifiers. They bind a new variable name to the symbolic logic statements. ∃ reads as there exists. ∀ reads for all so the first part of the statement would be read as:

forall x (of type 𝜎), there exists a y (of type 𝜏) such that ...

OTHER TIPS

$\forall$ reads as "for all", and $\exists$ reads as "there exists". So, in english we have

$$\text{"if }\underbrace{\text{for all $x$}}_{\forall x^\sigma}\text{ }\underbrace{\text{exists a $y$}}_{\exists y^\tau}\text{ with $R(x,y)$, }\underbrace{\text{then}}_\to\text{ }\underbrace{\text{there is a function $f$}}_{\exists f^{\sigma\to\tau}}\text{ so that }\underbrace{\text{for all $x$}}_{\forall x^\sigma}\text{ holds $R(x,f(x))$".}$$

I skipped over the $\sigma$ and $\tau$ superscripts, as they indicate types and are not of primary importance here.

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