Friday, March 27, 2009

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.


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!
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