The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A356190 a(n) is the least prime p such that there are exactly n primes strictly between p and 2*p, or -1 if there is no such p. 1

%I #9 Aug 18 2022 11:44:32

%S 2,7,11,17,23,29,31,-1,37,-1,53,61,59,71,79,89,-1,109,97,101,-1,-1,

%T 127,-1,137,151,149,157,-1,179,-1,191,-1,-1,211,-1,-1,223,233,-1,251,

%U 257,263,293,-1,-1,-1,-1,307,311,-1,-1,-1,331,349,347,367,373,379,389,409,-1,-1,419,-1,431,443

%N a(n) is the least prime p such that there are exactly n primes strictly between p and 2*p, or -1 if there is no such p.

%H Robert Israel, <a href="/A356190/b356190.txt">Table of n, a(n) for n = 1..9999</a>

%F a(n) <= A168421(n+1) - 2, with equality for n = 6, 263, 3061, 4750, 4893, 5029, 5555, 6101, ....

%e a(3) = 11 because there are exactly 3 primes between 11 and 22, namely 13, 17 and 19, and 11 is the least prime that works.

%p V:= Vector(100,-1): p:= 1:

%p for n from 1 while p < 727 do # note that A168421(101) = 727

%p p:= nextprime(p);

%p v:= numtheory:-pi(2*p)-n;

%p if v <= 100 and V[v] = -1 then

%p V[v]:= p;

%p fi

%p od:

%p convert(V,list);

%Y Cf. A168421.

%K sign

%O 1,1

%A _J. M. Bergot_ and _Robert Israel_, Jul 29 2022

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 29 04:26 EDT 2024. Contains 372921 sequences. (Running on oeis4.)