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A240878 Numbers n such that (n^2 + 2)/3 is prime. 2
2, 7, 11, 17, 29, 35, 37, 43, 53, 55, 65, 73, 79, 83, 97, 115, 119, 125, 133, 137, 155, 161, 169, 187, 191, 205, 209, 233, 251, 263, 269, 271, 277, 281, 287, 295, 335, 343, 359, 361, 379, 385, 389, 395, 407, 413, 425, 433, 451, 461, 475, 479, 493, 505, 511, 521, 529, 541, 559, 577 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n == 1 mod 3, then (n - 1)/3 is in A086285.

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

(2^2 + 2)/3 = 6/3 = 2, which is prime, so 2 is in the sequence.

(7^2 + 2)/3 = 51/3 = 17, which is prime, so 7 is in the sequence.

(11^2 + 2)/3 = 123/3 = 41, which is prime, so 11 is in the sequence.

(13^2 + 2)/3 = 171/3 = 57 = 3 * 19, which is not prime, so 13 is not in the sequence.

MAPLE

N:= 10000; # to get all terms <= 3 N + 2

A240878:= select(t -> isprime((t^2+2)/3), {seq(seq(3*i+j, j=1..2), i=0..N)}):

MATHEMATICA

Select[Range[500], PrimeQ[(#^2 + 2)/3] &] (* Alonso del Arte, Apr 13 2014 *)

PROG

(MAGMA) [2] cat [n: n in [4..600] | IsPrime((n^2 + 2) div 3)]; // Vincenzo Librandi, Jul 01 2014

(PARI) is(n)=isprime((n^2+2)/3) \\ Charles R Greathouse IV, Jun 06 2017

CROSSREFS

Cf. A086285.

Sequence in context: A075552 A274504 A124854 * A109953 A165810 A109417

Adjacent sequences:  A240875 A240876 A240877 * A240879 A240880 A240881

KEYWORD

nonn,easy

AUTHOR

Robert Israel, Apr 13 2014

STATUS

approved

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Last modified February 25 13:30 EST 2018. Contains 299654 sequences. (Running on oeis4.)