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Homesphenic number

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# sphenic number

Given three primes $p<q<r$, the composite integer $pqr$ is a sphenic number. The first few sphenic numbers are 30, 42, 66, 70, 78, 102, 105, 110, 114, 130, 138, 154, $\ldots$ listed in A007304 of Sloane’s OEIS.

The divisors of a sphenic number therefore are $1,p,q,r,pq,pr,qr,pqr$. Furthermore, $\mu(pqr)=(-1)^{3}$ (where $\mu$ is the Möbius function), $\tau(pqr)=8$ (where $\tau$ is the divisor function) and $\Omega(pqr)=\omega(pqr)=3$ (where $\Omega$ and $\omega$ are the number of (nondistinct) prime factors function and the number of distinct prime factors function, respectively).

The largest known sphenic number at any time is usually the product of the three largest known Mersenne primes.

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## Recent Activity

Oct 21

new question: Prime numbers out of sequence by Rubens373

Oct 7

new question: Lorenz system by David Bankom

Oct 19

new correction: examples and OEIS sequences by fizzie

Oct 13

new correction: Define Galois correspondence by porton

Oct 7

new correction: Closure properties on languages: DCFL not closed under reversal by babou

new correction: DCFLs are not closed under reversal by petey

Oct 2

new correction: Many corrections by Smarandache

Sep 28

new question: how to contest an entry? by zorba

new question: simple question by parag

new question: Prime numbers out of sequence by Rubens373

Oct 7

new question: Lorenz system by David Bankom

Oct 19

new correction: examples and OEIS sequences by fizzie

Oct 13

new correction: Define Galois correspondence by porton

Oct 7

new correction: Closure properties on languages: DCFL not closed under reversal by babou

new correction: DCFLs are not closed under reversal by petey

Oct 2

new correction: Many corrections by Smarandache

Sep 28

new question: how to contest an entry? by zorba

new question: simple question by parag