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A259788
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Greatest prime factor of phi(binomial(2*n,n)).
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3
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2, 2, 3, 3, 5, 5, 5, 5, 5, 3, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 7, 23, 23, 23, 23, 23, 23, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 83, 83, 83, 83, 83, 83, 89
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OFFSET
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2,1
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COMMENTS
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Conjectures:
(1) 7 is a unique term which is not a Sophie Germain prime (A005384);
(2) A Sophie Germain prime p occurs p times if and only if p=2,3,5 and 11; otherwise, it occurs q-p times, where q is the next Sophie Germain prime > p;
(3) a(n) is the greatest prime factor of p-1 for primes p in the interval (n, 2*n).
All these conjectures follow from the following strengthening of the Bertrand postulate for n>=24: the interval (n, 2*n) contains a safe prime (A005385).
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LINKS
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Peter J. C. Moses, Table of n, a(n) for n = 2..5001
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MATHEMATICA
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Map[First[Last[FactorInteger[EulerPhi[Binomial[2#, #]]]]]&, Range[2, 100]]
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CROSSREFS
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Cf. A000010, A000984, A005384, A005385, A006530, A066973.
Sequence in context: A308465 A276119 A167755 * A033302 A072729 A057872
Adjacent sequences: A259785 A259786 A259787 * A259789 A259790 A259791
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KEYWORD
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nonn
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AUTHOR
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Vladimir Shevelev, Jul 05 2015
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STATUS
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approved
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