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.
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.
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.
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.