Sphenic number

In mathematics, a sphenic number (Old Greek "sphen" = wedge) is a positive integer which is the product of three distinct prime numbers.

Note that this definition is more stringent than simply requiring the integer to have exactly three prime factors; e.g. 60 = 2² × 3 × 5 has exactly 3 prime factors, but is not sphenic.

All sphenic numbers have exactly eight divisors. If we express the sphenic number as $n\; =\; p\; cdot\; q\; cdot\; r$, where "p", "q", and "r" are distinct primes, then the set of divisors of "n" will be:

:$left\{\; 1,\; p,\; q,\; r,\; pq,\; pr,\; qr,\; n\; ight\}.$

All sphenic numbers are by definition squarefree, because the prime factors must be distinct.

The Möbius function of any sphenic number is −1.

The first few sphenic numbers are: 30, 42, 66, 70, 78, 102, 105, 110, 114, 130, 138, 154, 165, ... OEIS|id=A007304

The first case of two consecutive integers which are sphenic numbers is 230 = 2×5×23 and 231 = 3×7×11. The first case of three is 1309 = 7×11×17, 1310 = 2×5×131, and 1311 = 3×19×23. There is no case of more than three, because one of every four consecutive integers is divisible by 4 = 2×2 and therefore not squarefree.

As of|2008|9|url=http://primes.utm.edu/top20/page.php?id=3 the largest known sphenic number is (2^43,112,609 − 1) × (2^37,156,667 − 1) × (2^32,582,657 − 1), i.e., the product of the three largest known primes.

External links

* [http://www.research.att.com/projects/OEIS?Anum=A007304 Sphenic numbers] from On-Line Encyclopedia of Integer Sequences.