

A083371


Primes p such that qp >= 8, where q is the next prime after p.


6



89, 113, 139, 181, 199, 211, 241, 283, 293, 317, 337, 359, 389, 401, 409, 421, 449, 467, 479, 491, 509, 523, 547, 577, 619, 631, 661, 683, 691, 701, 709, 719, 743, 761, 773, 787, 797, 811, 829, 839, 863, 887, 911, 919, 929, 953, 983, 997, 1021, 1039, 1051, 1069
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OFFSET

1,1


COMMENTS

The original definition by Cloitre was: [Start from any initial value F(1) >= 2 and define F(n) as the largest prime factor of F(1)+F(2)+F(3)+...+F(n1). The sequence contains the primes satisfying F(2*p)=p supposed F(1)=7. Conjecture: F(n)= n/2+O(log n) and the sequence is infinite.] Don Reble showed Jan 22 2022 that these are the same primes p followed by a prime gap of qp >=8, where q is the next prime after p: [
Let X' be the first prime after X, 'X be the first prime before X.
The F sequence starting at "7" has 11 "7"s, then 6 "11"s, 6 "13"s, 6 "17"s, 6 "19"s, 10 "23"s, ...
One easily sees that the F sequence starting at prime S has S' instances of S; then for each prime P after S, it has (P''P) instances of P. (A076973 is the F sequence starting at "2".)
The primes from S to P occupy the first [S' + (S''S) + (S'''S') + ... + (P'  'P)] terms of F.
That sum telescopes to P'+PS, and so
F(P'+PS) = P; F(P'+PS+1) = P';
F(P+'PS) = 'P; F(P+'PS+1) = P.
If F(X) =P, then P+'PS < X <= P'+PS.
If F(2P)=P, then P+'PS < 2P <= P'+PS
'P < P+S <= P'
S <= P'P
So this sequence has the primes P for which P'P >= 7; and since P'P is even (both primes are odd), P'P >= 8. q.e.d.]


LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..1500
K. Soundararajan, Small gaps between prime numbers: the work of GoldstonPintzYildirim, Bull. Amer. Math. Soc., 44 (2007), 118.
Index entries for primes, gaps between


FORMULA

A000040 MINUS A124590.  R. J. Mathar, Jan 23 2022
A031926 UNION A031928 UNION A031930 UNION A031932 UNION ...  R. J. Mathar, Jan 23 2022


MAPLE

d:=8; M:=1000; t0:=[]; for n from 1 to M do p:=ithprime(n); if nextprime(p)  p >= d then t0:=[op(t0), p]; fi; od: t0; # N. J. A. Sloane, Dec 19 2006
f := proc(n) option remember: if(n=1)then return 7: fi: return max(op(numtheory[factorset](add(f(i), i=1..n1)))): end: seq(`if`(f(2*ithprime(n))=ithprime(n), ithprime(n), NULL), n=1..200); # Nathaniel Johnston, Jun 25 2011, via Cloitre's F


MATHEMATICA

Transpose[Select[Partition[Prime[Range[200]], 2, 1], Last[#]First[#]>7&]][[1]] (* Harvey P. Dale, Jan 28 2013 *)


CROSSREFS

Cf. A076973.
Sequence in context: A350724 A121608 A135144 * A124583 A257843 A155106
Adjacent sequences: A083368 A083369 A083370 * A083372 A083373 A083374


KEYWORD

nonn


AUTHOR

Benoit Cloitre, Jun 04 2003


EXTENSIONS

Terms after a(20) from Nathaniel Johnston, Jun 26 2011
Merged with A124583 in response to Reble's seqfan post.  R. J. Mathar, Jan 24 2022


STATUS

approved



