Simplest answer may be 3000: MMM or 4000: MMMM?
Wikipedia mentions the use of a horizontal overline to multiply by a thousand. Thus 4006: IVVI. Is this a palindrome? But then I suppose M would be 1000000 etc.
It also shows XX for 2,000,000 so I suppose M for 100,000,000 etc...
Could you do a double overline for billions etc.?
Incidentally, while searching a couple site show XIX = 19 as the largest prime that is a palindrome in roman numerals.

Posted by Jer
on 20150407 08:54:41 