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I hate when I come across symbols I can't recognise or describe because it's nigh-on impossible to google for them, so I hope it's okay to post this here. Does anyone know what this symbol means within the realm of Set Theory/Predicate Logic:

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In this context "WidgetNodule" is a state with the following specification:

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Andrés E. Caicedo
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Leon Aves
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  • Did you search list of math symbols and/or list of math symbols by subject ? – wltrup Aug 10 '15 at 21:01
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    Can you say more about the context in which you saw this? This is certainly not a standard notation broadly in "set theory/predicate logic". – Eric Wofsey Aug 10 '15 at 21:09
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    It almost seems like you are trying to obscure where this came from... Why not tell us the paper/book/whatever, or at least give a relatively complete example of how it is used. –  Aug 10 '15 at 21:26
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    I couldn't agree more with the above comments about the context. This notation is very familiar to those in the know about Z (see my answer), but there are probably very few such people in the MSE community and it would have been a kindness to the rest of the community to say where you came across your problem. – Rob Arthan Aug 10 '15 at 22:15

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This is in the realm of formal specification in computer science: the notation you are looking at is the Z specification language. Z is used for specifying systems by describing their states and state transitions in (a typed) set theory. In Z, $\Xi S$ is an abbreviation meaning that the part $S$ of the state is unchanged by a transition: in this case $\Xi\mathit{WidgetNodule}$ encapsulates the equation $\mathit{seen}' = \mathit{seen}$.

Rob Arthan
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