905

905 = 5 x 181.

Most odd integers can be written as the sum of a prime and a power of two in at least one way. Ones that cannot are called obstinate numbers. They form an infinite set but are not all that common. The first composite obstinate number is 905. So 905 is the smallest composite number that is not the sum of a prime and a power of two.

905 is the sum of seven consecutive primes: 109 + 113 + 127 + 131 + 137 + 139 + 149 = 905.

905 has two representations as a sum of two squares: 905 = 8² + 29² = 11² + 28².

905 is the hypotenuse of two Pythagorean triples: 905² = 464² + 777² = 616² + 663².


A Neptune-Pluto conjunction occurred in Taurus in the year 905.

Source: Prime Curios!