It is common to use proper names as adjectives. However, the right use is not always clear to me. To illustrate the question, I'm considering the examples below:
- In Newton's theory, objects have mass.
- In the Newton theory, objects have mass.
- In Newton theory, objects have mass.
According to me, 1. and 2. are correct, above. I am not sure about 3. Are there systematic rules on this matter? Also, there might be additional difficulties when a sentence is started with the proper name:
- Newton theory was developed...
- The Newton theory was developed...
I am undecided. We could also have an additional adjective as follows:
- The preferred Newton assumptions are...
- Preferred Newton's assumptions are...
Not to be confused with Newton's preferred assumptions.
[EDIT] What happens when two proper names are combined, with for instance, something like:
- In the Lebesgue-Kolmogorov theory
- In Lebesgue-Kolmogorov's theory