The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A092938 a(n) = least prime p such that 2*prime(n) - p is prime. 3
 2, 3, 3, 3, 3, 3, 3, 7, 3, 5, 3, 3, 3, 3, 5, 3, 5, 13, 3, 3, 7, 7, 3, 5, 3, 3, 7, 3, 7, 3, 3, 5, 3, 7, 5, 19, 3, 13, 3, 29, 5, 3, 3, 3, 5, 19, 3, 3, 5, 19, 3, 11, 3, 3, 5, 3, 17, 19, 7, 5, 3, 17, 7, 3, 7, 3, 3, 13, 3, 7, 5, 17, 7, 3, 7, 5, 5, 7, 5, 7, 11, 3, 3, 3, 19, 3, 11, 3, 3, 7, 5, 5, 3, 5, 7, 23, 5, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(n) = least prime p such that prime(n) = (p+q)/2, where q is also prime. a(n) <= prime(n). Conjecture: a(n) = prime(n) only for n = 1 and 2. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 2*prime(8) = 38; 38 - 2 = 36, 38 - 3 = 35, 38 - 5 = 33 are composite, but 38 - 7 = 31 is prime. Hence a(8) = 7. MAPLE f:= proc(n) local pn, p;    pn:= ithprime(n);    p:= 1;    do      p:= nextprime(p);      if isprime(2*pn-p) then return p fi    od end proc: map(f, [\$1..100]); # Robert Israel, Jul 31 2020 PROG (PARI) {for(n=1, 98, k=2*prime(n); p=2; while(!isprime(k-p), p=nextprime(p+1)); print1(p, ", "))} \\ Klaus Brockhaus, Dec 23 2006 CROSSREFS Cf. A092939, A092940, A116619. Sequence in context: A035441 A025784 A035390 * A320110 A068953 A189635 Adjacent sequences:  A092935 A092936 A092937 * A092939 A092940 A092941 KEYWORD nonn AUTHOR Amarnath Murthy, Mar 23 2004 EXTENSIONS Edited and extended by Klaus Brockhaus, Dec 23 2006 STATUS approved

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

Last modified May 11 11:54 EDT 2021. Contains 343791 sequences. (Running on oeis4.)