Find the image of a given value in a relation

Question

I have a relation R given below:

R = {(a,1), (a,2), (b,1)}

To get the image set of a, we can write R(|{a}|) = {1,2} in Z notation, using thick brackets symbol (| |).

Is there a mathematical symbol to represent this rather than using the Z notation? Can we have R(a) like what we usually do to get a function image of a given value, e.g. f(a)?

Answer 1

In mathematics, using brackets is common: R[{a}] = {1,2}

See also math. image in Wikipedia.