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A084920
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a(n) = (prime(n)-1)*(prime(n)+1).
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33
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3, 8, 24, 48, 120, 168, 288, 360, 528, 840, 960, 1368, 1680, 1848, 2208, 2808, 3480, 3720, 4488, 5040, 5328, 6240, 6888, 7920, 9408, 10200, 10608, 11448, 11880, 12768, 16128, 17160, 18768, 19320, 22200, 22800, 24648, 26568, 27888, 29928
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OFFSET
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1,1
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COMMENTS
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Integers k for which there exist exactly two positive integers b such that (k+1)/(b+1) is an integer. - Benedict W. J. Irwin, Jul 26 2016
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LINKS
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FORMULA
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a(n) = (1/2) * Sum_{|k|<=2*sqrt(p)} k^2*H(4*p-k^2) where H() is the Hurwitz class number and p is n-th prime. - Seiichi Manyama, Dec 31 2017
Product_{n>=1} (1 + 1/a(n)) = Pi^2/6 (A013661).
Product_{n>=1} (1 - 1/a(n)) = A065469. (End)
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MAPLE
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MATHEMATICA
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PROG
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(Haskell)
a084920 n = (p - 1) * (p + 1) where p = a000040 n
(Sage) [(p-1)*(p+1) for p in primes(200)] # Bruno Berselli, Mar 30 2015
(PARI) a(n) = (prime(n)-1)*(prime(n)+1); \\ Michel Marcus, Jul 28 2016
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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