2047

This number is a composite.

The smallest composite Mersenne number with a prime exponent (2¹¹ - 1 = 23 x 89).

The smallest strong base-2 pseudoprime. [Terr]

The 16th century French philosopher, Charles de Bouvelles, perhaps due to difficulties in division, cited falsely 2047 as prime.

The smallest Woodall number (2047 = 8*2^8-1) with a Woodall prime as its smallest factor.

The smallest Mersenne semiprime. [Gupta]

The next brilliant number year (23 * 89). [Gaydos]