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A028871 Primes of the form n^2 - 2. 35
2, 7, 23, 47, 79, 167, 223, 359, 439, 727, 839, 1087, 1223, 1367, 1847, 2207, 2399, 3023, 3719, 3967, 4759, 5039, 5623, 5927, 7919, 8647, 10607, 11447, 13687, 14159, 14639, 16127, 17159, 18223, 19319, 21023, 24023, 25919, 28559, 29927 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Except for the initial term, primes equal to the product of two consecutive even numbers minus 1. - Giovanni Teofilatto, Sep 24 2004

With exception first term 2, primes p such that continued fraction of (1+Sqrt[p])/2 have period 4. [Artur Jasinski, Feb 03 2010]

REFERENCES

D. Shanks, Solved and Unsolved Problems in Number Theory, 2nd. ed., Chelsea, 1978, p. 31.

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

P. De Geest, Palindromic Quasipronics of the form n(n+x)

Eric Weisstein's World of Mathematics, Near-Square Prime

EXAMPLE

a(3) = 23 = 5^2 - 2 = A028870(3)^2 - 2.

MATHEMATICA

lst={}; Do[s=n^2; If[PrimeQ[p=s-2], AppendTo[lst, p]], {n, 6!}]; lst (* Vladimir Joseph Stephan Orlovsky, Sep 26 2008 *)

aa = {}; Do[If[4 == Length[ContinuedFraction[(1 + Sqrt[Prime[m]])/2][[2]]], AppendTo[aa, Prime[m]]], {m, 1, 1000}]; aa (* Artur Jasinski, Feb 03 2010 *)

Select[Table[n^2 - 2, {n, 400}], PrimeQ] (* Vincenzo Librandi, Jun 19 2014 *)

PROG

(PARI) list(lim)=select(n->isprime(n), vector(sqrtint(floor(lim)+2), k, k^2-2)) \\ Charles R Greathouse IV, Jul 25 2011

(Haskell)

a028871 n = a028871_list !! (n-1)

a028871_list = filter ((== 1) . a010051') a008865_list

-- Reinhard Zumkeller, May 06 2013

(MAGMA) [p: p in PrimesUpTo(100000)| IsSquare(p+2)]; // Vincenzo Librandi, Jun 19 2014

CROSSREFS

Cf. A028870, A089623, A010051; subsequence of A008865.

Sequence in context: A049552 A049572 A094786 * A053705 A049001 A049002

Adjacent sequences:  A028868 A028869 A028870 * A028872 A028873 A028874

KEYWORD

nonn,easy

AUTHOR

Patrick De Geest

STATUS

approved

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Last modified November 22 20:13 EST 2014. Contains 249827 sequences.