It is an interesting fact, though, since there is a sequence of integers called "integers factored by computers". This integer is now a member of that set. The fact that a number this large was factored is interesting to the mathematical community. Should this information be suppressed because TI used this number to sign their OS?

What if there was a table of all numbers and their factorizations, 1 = 1, 2 = 2, 3 = 3, 4 = 2 * 2, ... If you noticed TI was using a number in this list as their signing key, you could just look it up. Would that be illegal?

Then why is it illegal to lazily generate this list as needed?

What other properties of integers are illegal to discuss?

(FWIW, I am almost certain that the DMCA does not apply to this case, anyway. TI is just sending letters Because They Can and because there is no penalty if they're wrong. I can send you a C&D telling you to take down the above comment. The appropriate response is to mock me, not to take the comment down.)

Basically, TI should have used a bigger integer. Why rely on something as flaky as the legal system to protect your business model when you can use mathematical fact instead? (Answer: because using a bigger number would have added 1 cent to the cost of every calculator sold. Sending C&Ds is free.)