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A225861 Numbers n such that (n + prime(n))/2 is a prime. 2
15, 19, 21, 23, 25, 31, 37, 39, 57, 67, 75, 77, 83, 99, 107, 111, 115, 145, 165, 175, 189, 195, 207, 235, 237, 247, 273, 293, 303, 311, 315, 337, 351, 353, 361, 395, 405, 439, 445, 471, 477, 493, 495, 499, 537, 545, 555, 567, 569, 581, 585, 591, 619, 631 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Average of n and n-th prime is a prime. All n must be odd.

LINKS

Paolo P. Lava, Table of n, a(n) for n = 1..1000

EXAMPLE

15 is in the list since (15+Prime(15))/2=31.

MATHEMATICA

Select[Range[632], PrimeQ[(# + Prime[#])/2] &]

CROSSREFS

Cf. A000040.

Sequence in context: A217406 A181664 A224535 * A329184 A077349 A257020

Adjacent sequences:  A225858 A225859 A225860 * A225862 A225863 A225864

KEYWORD

nonn

AUTHOR

Jayanta Basu, May 18 2013

STATUS

approved

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Last modified September 23 12:43 EDT 2021. Contains 347617 sequences. (Running on oeis4.)