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A125685 Primes in A125683(n) = numerator[ Sum[ (-1)^(k+1) * 1/(k(k+1)), {k,1,n} ]. 3
5, 11, 2, 79, 331, 479, 5297, 70061, 69203, 8960447, 45083347, 1031626241, 15484789693, 15537907043, 64166447971, 3979714828967, 3988907823167, 27918983997629, 598858179567591121853, 31710728461561839214229 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Corresponding numbers n such that A125683(n) is prime are listed in A125684(n) = {3,4,5,6,7,8,10,13,14,18,21,22,26,27,28,32,33,35,51,54,58,67,76,89,100,...}.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..65

FORMULA

a(n) = A125683[ A125684(n) ].

EXAMPLE

A125683(n) begins {1,1,5,11,2,79,331,479,493,5297,2701,69071,70061,...}.

Thus a(1) = 5 because A125683(3) = 5 is prime but A125683(k) is not prime for k<3.

a(2)-a(6) = {11,2,79,331,479} because A125683(k) is prime for 3<k<9.

MATHEMATICA

Select[Table[Numerator[Sum[(-1)^(k+1)*1/(k(k+1)), {k, 1, n}]], {n, 1, 100}], PrimeQ]

CROSSREFS

Cf. A125683, A125684.

Sequence in context: A111118 A159199 A125683 * A098147 A100298 A066461

Adjacent sequences:  A125682 A125683 A125684 * A125686 A125687 A125688

KEYWORD

nonn

AUTHOR

Alexander Adamchuk, Nov 30 2006

STATUS

approved

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Last modified November 19 06:26 EST 2019. Contains 329310 sequences. (Running on oeis4.)