239 (two hundred [and] thirty-nine) is the natural number following 238 and preceding 240. Properties 239 is a prime number. The next is 241, with which it forms a pair of twin primes; hence, it is also a Chen prime. 239 is a Sophie Germain prime and a Newman–Shanks–Williams prime. It is an Eisenstein prime with no imaginary part and real part of the form 3n − 1 (with no exponentiation implied). 239 is a factor of the repdigit 1111111, with the other prime factor being 4649. 239 is also a happy number.
239 is the only number besides 23 that requires 9 cubes sum to it, see Waring problem, 239 also requires the largest possible number (19) of fourth powers sum to it.
239 is the only number n besides 1 such that (n2+1)/2 is a fourth power, see Ljunggren's equation.
239 is the smallest positive integer d such that the imaginary quadratic field Q() has class number = 15.
239 is the smallest number that contains the highest possible digit in all bases from 2 to 12: 111011112 222123 32334 14245 10356 4617 3578 2859 23910 1A811 17B12
The next number with this property is 5927.
HAKMEM entry HAKMEM (incidentally AI memo 239 of the MIT AI Lab) included an item on the properties of 239, including these: When expressing 239 as a sum of square numbers, 4 squares are required, which is the maximum that any integer can require; it also needs the maximum number (9) of positive cubes (23 is the only other such integer), and the maximum number (19) of fourth powers. 239/169 is a convergent of the simple continued fraction of the square root of 2, so that 2392 = 2 · 1692 − 1. Related to the above, = 45°. 239 · 4649 = 1111111, so 1/239 = 0.0041841 repeating, with period 7. 239 can be written as bn − bm − 1 for b = 2, 3, and 4, a fact evidenced by its binary representation 11101111, ternary representation 22212, and quaternary representation 3233. There are 239 primes < 1500. 239 is the largest integer n whose factorial can be written as the product of distinct factors between n + 1 and 2n, both included. The only solutions of the Diophantine equation y2 + 1 = 2x4 in positive integers are (x, y) = (1, 1) or (13, 239).
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