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A089765
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Composite n whose sum of distinct divisors, s(d), ignoring divisors n and 1, divided by the count of divisors (not counting n and 1), c(d), are primes. Duplicate divisors, as in 2*2=4 are counted just once.
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4
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4, 8, 9, 18, 21, 25, 33, 49, 57, 69, 81, 85, 93, 121, 129, 133, 145, 169, 177, 205, 213, 217, 237, 249, 253, 265, 273, 289, 309, 361, 393, 417, 445, 469, 489, 493, 505, 517, 529, 553, 565, 573, 597, 633, 669, 685, 697, 753, 777, 781, 793, 813, 817, 841, 865
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OFFSET
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1,1
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REFERENCES
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Glenn James and Robert C. James, Mathematics Dictionary, Princeton, N.J.: D. Van Nostrand Co., Inc., 1959; page 154 (factor of an integer).
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LINKS
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FORMULA
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Factor n into its distinct divisors, ignore n and 1, add the divisors and divide by the number of divisors. If s(d) / c(d) [sum divided by count] is prime, add to sequence.
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EXAMPLE
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a(1)= 8 because its factors are 8, 1, 2, 4. Ignoring 8 and 1, the sum of 2+4=6. The count of factors is 2 and 6/2=3, a prime.
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MATHEMATICA
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aQ[n_] := CompositeQ[n] && PrimeQ[(DivisorSigma[1, n] - n - 1)/(DivisorSigma[0, n] - 2)]; Select[Range[865], aQ] (* Amiram Eldar, Sep 07 2019 *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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