Como dizer "É por isso que dentro da..." em inglês

As theory X goes...
As far as theory X goes...
According to theory X, ...

Would do, for the time being. There are other ways, let's wait for further comments.

É por isso que dentro da teoria X, acredita-se que Y + Z é igual a G ?

Here a more 'mathematical' way to formulate the sentence, like finishing an oral explanation.
That's why, as stated by theory X, Y + Z equals G.

The above sentence would amount to:
That's the proof, as stated by theory X, that Y + Z equals G.
That confirms that, as per theory X, that Y + Z equals G. (OR that is the confirmation that Y + Z equals G, as theory X goes.)
That's my/the reasoning behind (OR support/back up) the theory X, it (the reasoning) confirms that Y + Z equals G.

Of course, I took a more 'language approach' than a mathematical one, and of course a K12 estudent might be terser:
Perhaps would put something like "X, Y + Z equals G is solved as the following - using the theory X" in wrting.
It's a stretch, a lot of guesswork on my part, since I am no teacher of mathematics, let alone in English.
But, not being much technical it's a translation of sorts. That could just answer your question, not to be used professionally in the classroom to teach English-speaking people. See?