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Search: id:A123751
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| 5, 266681, 40799043101, 86364397717734821, 36190908596780862323291147613117849902036356128879432564211412588793094572280300268379995976006474252029, 334279880945246012373031736295774418479420559664800307123320901500922509788908032831003901108510816091067151027837158805812525361841612048446489305085140033
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OFFSET
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1,1
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COMMENT
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A007406[n] are the Wolstenholme numbers: numerator of Sum 1/k^2, k = 1..n. Numbers n such that A007406[n] is prime are listed in A111354[n] = {2,7,13,19,121,188,252,368,605,745,1085,1127,1406,...}.
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LINKS
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Eric Weisstein's World of Mathematics, Harmonic Number.
Eric Weisstein's World of Mathematics, Wolstenholme's Theorem.
Eric Weisstein's World of Mathematics, Wolstenholme Number
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FORMULA
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a(n) = A007406[ A111354[n] ].
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EXAMPLE
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A007406[n] begins {1, 5, 49, 205, 5269, 5369, 266681, 1077749, 9778141, ...}.
Thus a(1) = 5 because A007406[2] = 5 is prime but A007406[1] = 1 is not prime.
a(2) = 266681 because A007406[7] = 266681 is prime but all A007406[k] are composite for 2 < k < 7.
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MATHEMATICA
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Do[f=Numerator[Sum[1/i^2, {i, 1, n}]]; If[PrimeQ[f], Print[{n, f}]], {n, 1, 250}]
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CROSSREFS
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Cf. A111354, A007406, A001008, A007407, A067567, A056903.
Sequence in context: A133381 A038027 A057679 this_sequence A067502 A067509 A123693
Adjacent sequences: A123748 A123749 A123750 this_sequence A123752 A123753 A123754
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KEYWORD
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nonn
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AUTHOR
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Alexander Adamchuk (alex(AT)kolmogorov.com), Oct 11 2006
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