What is undefined times zero? Einstein's energy equation (after substituting the equation of relativistic momentum) takes this form:
$$E =   \frac{1}{{\sqrt {1 - {v^2}/{c^2}} }}{m_0}{c^2}
% $$
Now if you apply this form to a photon (I know this is controversial, in fact I would not do it, but I just want to understand the consequences), you get the following:
$$E = \frac{1}{0}0{c^2}% $$
On another note, I understand that after dividing by zero:


*

*If the numerator is any number other than zero, you get an "undefined" = no solution, because you are breaching mathematical rules.

*If the numerator is zero, you get an "indeterminate" number = any value.


Here it seems we would have an "indeterminate" [if (1/0) times 0 equals 0/0], although I would prefer to have an "undefined" (because I think that applying this form to a photon breaches physical/logical rules, so I would like the outcome to breach mathematical rules as well...) and to support this I have read that if a subexpression is undefined (which would be the case here with gamma = 1/0), the whole expression becomes undefined (is this right and if so does it apply here?).
So what is the answer in strict mathematical terms: undefined or indeterminate?

Undefined is not a number. There is no such number as undefined, for which you could define the multiplication operation.
You could extend a set of a numbers (the set of the real numbers or the set of the complex numbers) with a new element, what you call "undefined", and then define a multiplication on this set as usual. It might be possible (although there are major problems solve).
However, from this moment, you have a defined value what you still call "undefined". Well, paper can hold everything, but it does not really sound as useful mathematics.
