641

This number is a prime.

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The largest 3-digit number n such that (10^n+1)/11 is prime. [Loungrides]

The only non-titanic prime of form 2^x+5^x, for x a composite number (x=4). [Loungrides]

The sum of all the palindromes from 1 up to 101 is prime, i.e., 1+2+3+...+9+11+22+33+...+99+101 = 641. [Loungrides]