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 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

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