Weird number

The term "weird number" also refers to a phenomenon in two's complement arithmetic.

In number theory, a weird number is a natural number that is abundant but not semiperfect. In other words, the sum of the proper divisors (divisors including 1 but not itself) of the number is greater than the number, but no subset of those divisors sums to the number itself.

Examples

The smallest weird number is 70. Its proper divisors are 1, 2, 5, 7, 10, 14, and 35; these sum to 74, but no subset of these sums to 70. The number 12, for example, is abundant but not weird, because the proper divisors of 12 are 1, 2, 3, 4, and 6, which sum to 16; but 2+4+6 = 12.

The first few weird numbers are

70, 836, 4030, 5830, 7192, 7912, 9272, 10430, 10570, 10792, 10990, 11410, 11690, 12110, 12530, 12670, 13370, 13510, 13790, 13930, 14770, ... (sequence A006037 in OEIS).

Properties

Open problem in mathematics: Are there any odd weird numbers? (more open problems in mathematics)

It has been shown that an infinite number of weird numbers exist; in fact, the sequence of weird numbers has positive asymptotic density.

It is not known if any odd weird numbers exist; if any do, they must be greater than 2³² ≈ 4×10⁹ or 1×10¹⁷

Sidney Kravitz has shown that for k a positive integer, Q a prime exceeding 2^k, and

$R=\frac{2^kQ-(Q+1)}{(Q+1)-2^k}$;

also prime and greater than 2^k, then

$n=2^{k-1}QR$

is a weird number. With this formula, he found a large weird number

$n=2^{56}\cdot(2^{61}-1)\cdot153722867280912929\ \approx\ 2\cdot10^{52}$.

Primitive weird numbers

A property of weird numbers is that if n is weird, and p is a prime greater than the sum of divisors σ(n), then pn is also weird. This leads to the definition of primitive weird numbers, i.e. weird numbers that are not multiple of other weird numbers. Indeed, the construction of Kravitz allows us to build primitive weird numbers. It is conjectured that there exist infinitely many primitive numbers, and Melfi has shown that the infiniteness of primitive weird numbers is a consequence of Cramér's conjecture

External links

• Weisstein, Eric W., "Weird number", MathWorld.