18

This number is a composite.

Just showing those entries submitted by 'Bajpai': (Click here to show all)

The largest value of n less than ten thousand such that (3*2^n-1) and (2*2^n-1) are both prime. [Bajpai]

18 times the 18th prime has the sum of digits equal to 18. It is the largest such number with this property. [Bajpai]