wtf! 
Discussion
Saved to my thinkbank.
I understand the formula but not the implications, other than determining whether a given number is rational 😅
The case when x is rational is easier to see as m!x will be an integer from some value of m.
Cool formula 🤓
The other side, I guess the cos^2 is less than 1 so the limit is 0.
I choose many server to save my images
I can live with it.
I mean … yeah …
Can x at least be reasonable?
ITS ALL GREEK TO ME :)
If X is rational, there will be at least one (actually, infinitely many) Ms for which MX is an integer, and therefore the cosine is either -1 or +1
M! includes many even numbers, therefore the cosine will be +1, not -1
For irrational X, the cosine is always strictly between -1 and +1, and therefore tends to zero as n increases
(I still suspect in missing something, no pen and paper at hand)
And now I'm curious about complex X too