241

This number is a prime.

Let U(0) = 0, U(1) = 1, U(n) = 3*U(n-1) + U(n-2) for n>1, then 241 is the only known prime p such that p^2 divides U(p-(13|p)), where (13|p) is the Legendre symbol, i.e., a Wall-Sun-Sun-type prime to this Lucas sequence. [Luen]