A109628 - OEIS

A109628

Numbers k such that the numerator of Sum_{i=1..k} 1/prime(i), in reduced form, is prime.

2, 3, 5, 6, 18, 19, 22, 47, 57, 58, 63, 70, 73, 112, 632, 1382, 4621

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OFFSET

1,1

COMMENTS

Terms <= 112 correspond to certified primes.

Numbers k such that the arithmetic derivative of the k-th primorial, A024451(k) [= A003415(A002110(k))] is prime. - Antti Karttunen, Jan 09 2024

LINKS

Table of n, a(n) for n=1..17.

EXAMPLE

Sum_{i=1..6} 1/prime(i) = 40361/30030 and 40361 is prime, hence 6 is a term.

MATHEMATICA

s = 0; Do[s += 1/Prime[n]; k = Numerator[s]; If[PrimeQ[k], Print[n]], {n, 1, 1500}]

Position[Accumulate[1/Prime[Range[120]]], _?(PrimeQ[ Numerator[ #]]&)] //Flatten (* To generate terms greater than 120, increase the Range constant, but the program may take much longer to run. *) (* Harvey P. Dale, Jan 01 2019 *)

CROSSREFS

Cf. A002110, A003415.

Positions of primes in A024451.

Sequence in context: A126250 A293027 A276353 * A192367 A227230 A102977

Adjacent sequences: A109625 A109626 A109627 * A109629 A109630 A109631

KEYWORD

hard,more,nonn

AUTHOR

Ryan Propper, Aug 02 2005

EXTENSIONS

a(17) from Michael S. Branicky, Sep 16 2024

STATUS

approved