261

261 = 3² x 29.

261 has a representation as a sum of two squares: 261 = 6² + 15².

261 is the only three-digit number n for which 2ⁿ - n is a prime number.

261 is a nonagonal number.

Inserting the digit 1 between adjacent digits of 261 gives a prime: 21611.

In the novel Contact by Carl Sagan, Dr. Arroway asks Mr. Kitz, "You wanted to keep the first 261 prime numbers secret?"

Source: Prime Curios!