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Is everything that you can write in math (that makes mathematical sense) an expression? If not, what would be examples of non-expressions? And would all expressions be composed of expressions themselves?

Also, are operators (like the differential operator) by themselves expressions?

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    What do you mean by "expression"?2011-01-16
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    @Harry whatever mathematicians like to use it for.2011-01-16
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    @wrongusername: the term does not, to my knowledge, have a fixed meaning in mathematics.2011-01-16
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    @Qiaochu oh ok, nevermind then. thanks for clearing that up :P2011-01-16
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    Look up http://en.wikipedia.org/wiki/Well-formed_formula for an example of defining what an "expression" is in one part of mathematics.2011-01-16
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    [Everything in Mathematica is an expression](http://reference.wolfram.com/mathematica/tutorial/EverythingIsAnExpression.html)...2011-01-17

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