

A078587


Largest prime p such that p<n and 2np is also prime.


8



3, 3, 5, 3, 5, 7, 7, 5, 11, 7, 11, 13, 13, 11, 17, 7, 17, 19, 13, 17, 19, 19, 23, 23, 19, 17, 29, 19, 23, 29, 31, 29, 31, 31, 29, 37, 37, 29, 41, 19, 41, 43, 31, 41, 43, 37, 47, 43, 43, 47, 47, 43, 53, 53, 43, 47, 59, 43, 53, 59, 61, 59, 61, 61, 53, 67, 67, 59, 71, 67, 59, 71
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

4,1


COMMENTS

Suggested by Goldbach Conjecture.
Also, values of p from A143697. This follows from the factorization n^2k^2 = (nk)(n+k).  T. D. Noe, Jan 22 2009


LINKS

P. CAMI, Table of n, a(n) for n = 4..60000


FORMULA

a(n) = 2n  A078496(n)


MATHEMATICA

Table[p=n+1; q=2np; While[q>0&&!(PrimeQ[p]&&PrimeQ[q]), p++; q ]; q, {n, 4, 100}]


PROG

PARI) a(n) = {my(p = precprime(n1)); while(!isprime(2*np), p = precprime(p1)); p; } \\ Michel Marcus, Oct 22 2016


CROSSREFS

Cf. A143697, A078496.
Cf. A082467
Sequence in context: A204022 A131832 A255316 * A239931 A033558 A046217
Adjacent sequences: A078584 A078585 A078586 * A078588 A078589 A078590


KEYWORD

easy,nonn


AUTHOR

T. D. Noe, Dec 02 2002


EXTENSIONS

Edited by N. J. A. Sloane, Jan 24 2009 at the suggestion of R. J. Mathar and T. D. Noe.


STATUS

approved



