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We use letters for unknowns/variables:

$x^2=4$

Are there variables/unknowns for operations too?

$8 \star 7 $

With the $\star $ being any operation.

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    I'm not very sure about the notation of these variables, supose I'm going to use a large number of variables, what should I do? $\star1,\star2,\star3...$? Of course I can invent a method, but I'm searching for common notation, so that people can understand what I mean.2012-07-08

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Of course. For example, the Cayley-Hamilton theorem states that, if $a_nx^n+\cdots+a_1x+a_0$ is the characteristic polynomial of a linear operator $M$, then $M$ is a root of $a_nX^n+\cdots+a_1X+a_0$ where $X$ is a variable representing a linear operator (often called a matrix). A less common example (but probably more in the spirit of your question) is the Eckmann-Hilton argument, which shows that any two binary operators $\cdot$ and $\star$ which satisfy certain conditions are equivalent.

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    I'll the new question anyway - I'll just point the operators with text.2012-07-08
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Sure, but $\circ$ isn't a good choice; usually it denotes function composition. I would use $\star$, for example, which doesn't have an existing widely-used meaning.

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    I switched it as you suggested.2012-07-08