There are things called e-mail correspondence, articles in libraries, articles on the Internet, etc.
The “Dr Ismail AK” from Turkey theory is possible, but frankly I find that a bit far-fetched, although not as far-fetched as the “Moby Dick” or “James Fenimore Cooper” theories.
We have no reason to believe that all of the police (and I’ll bet there would have been at least a few who took a look at the body and saw the words) misread “Ismail AK” to be “Ismail AX,” but it’s not inconceivable.
But why would he write it on his arm? If it was just a doctor he was communicating with by email or whose articles he was reading, that makes little sense.
I think the most likely explanation to my mind at this point in time is that it was his gamer ID and he put it on his arm as sort of an identity tattoo.
Or, maybe he had been flirting with islam and had taken that as his new name and put it on his arm as a sort of a brand.
If not that, maybe it was a newly-acquired computer password of some sort, and he wrote it on his arm to remember it, although it doesn’t seem like such a hard password to remember.