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A153357
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Numbers n such that the harmonic number numerator A001008(n) is a semiprime.
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1
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4, 6, 11, 14, 15, 17, 19, 20, 23, 25, 31, 33, 34, 35, 37, 39, 49, 53, 55, 59, 61, 68, 90, 93, 94, 101, 116, 117, 121, 124, 145, 155, 158, 163, 169, 170, 186, 193, 194, 199, 205, 211, 214, 245, 258, 259, 264, 267, 283, 311, 315, 328, 340, 347, 359, 365, 371, 385
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OFFSET
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1,1
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COMMENTS
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414, 421, 425, 436, 451, 452, and 480 are in the sequence. 391 and 476 are the remaining candidates below 500. - Daniel M. Jensen, Jun 26 2020
Numerator(H_391) is fully factored and confirmed semiprime with the help of NFS@Home. - Tyler Busby, May 06 2024
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REFERENCES
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R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics. Addison-Wesley, Reading, MA, 1990, p. 259.
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, page 347.
D. E. Knuth, The Art of Computer Programming. Addison-Wesley, Reading, MA, Vol. 1, p. 615
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LINKS
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CROSSREFS
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Cf. A001008 (numerators of harmonic number H(n)=Sum_{i=1..n} 1/i).
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KEYWORD
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hard,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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