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A092938 a(n) = least prime p such that 2*prime(n) - p is prime. 2
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

Table of n, a(n) for n=1..98.

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.

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

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Last modified April 5 12:34 EDT 2020. Contains 333241 sequences. (Running on oeis4.)