login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A247079 Numbers for which the harmonic mean of nontrivial divisors is an integer and which are not a square of prime numbers. 2
345, 1050, 1645, 4386, 6489, 8041, 13026, 23881, 88473, 115957, 255041, 342637, 377201, 1497517, 2132021, 2428489, 3256261, 3847001, 4114285, 4646101, 5054221, 6816865, 7218641, 7587901, 13384681, 14872837, 17897737, 20901553, 23807821, 25863653, 28207957 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

That's the numbers which are in A247078 and not in A001248.

a(149) >= 2*10^11. - Hiroaki Yamanouchi, Nov 20 2014

LINKS

Daniel Lignon and Hiroaki Yamanouchi, Table of n, a(n) for n = 1..148 (terms a(1)-a(40) from Daniel Lignon)

EXAMPLE

345 is not the square of a prime number and the nontrivial divisors of 345 are [3,5,15,23,69,115]. Their harmonic mean is 6/(1/3+1/5+1/15+1/23+1/69+1/115)=9.

PROG

(PARI) isok(n) = !(issquare(n) && isprime(sqrtint(n))) && (d=divisors(n)) && (#d > 2) && (denominator((#d-2)/sum(i=2, #d-1, 1/d[i])) == 1); \\ Michel Marcus, Nov 17 2014

CROSSREFS

Cf. A001599 (harmonic numbers), A247078.

Sequence in context: A116347 A063370 A095963 * A236731 A138043 A048904

Adjacent sequences:  A247076 A247077 A247078 * A247080 A247081 A247082

KEYWORD

nonn

AUTHOR

Daniel Lignon, Nov 17 2014

EXTENSIONS

a(15)-a(24) from Michel Marcus, Nov 17 2014

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 12 10:19 EST 2019. Contains 329953 sequences. (Running on oeis4.)