My question goes from Phys.SE where people advised me to ask my question here.
I always learn in maths and physics when something is a constant in an equation we have to put it before which varies. Some days ago I just thought about it.
When you say a function is linear you say something like, let $f$ be a linear function such as $f(x)=ax$. ...
Now, why, when we say the energy is a linear function of the mass we write, $$E=mc^2$$
Instead of, $$E=c^2 m$$ because light speed is a constant.
Is this something aesthetic, does Einstein wrote it like that and then everybody does? Why don't we respect the convention when we do relativity?
My question is more general because if we look closely to GR equations when there are product terms with the speed of light then the speed of light is always the second. For example we also write $$E^2=m^2c^4+p^2c^2$$ which should be written as $$E^2=c^4m^2+c^2p^2$$
And there are many others.