A090697 Numbers n such that n^2/2 - 1 is a prime.

4, 6, 8, 12, 14, 16, 20, 22, 26, 30, 34, 36, 42, 44, 48, 50, 56, 68, 72, 76, 78, 82, 84, 86, 90, 92, 98, 100, 104, 112, 118, 124, 126, 128, 138, 146, 152, 160, 162, 170, 174, 182, 184, 190, 196, 204, 216, 218, 224, 226, 230, 236, 250, 252, 254, 264, 268, 274, 280

Offset: 1

Author

Giovanni Teofilatto, Dec 20 2003

Keywords

• easy
• nonn

Comments

A066436 gives resulting primes p such that 2p+2 is square. - Ray Chandler, Dec 25 2003

References

• M. Cerasoli, F. Eugeni and M. Protasi, Elementi di Matematica Discreta, Bologna 1988
• Emanuele Munarini and Norma Zagaglia Salvi, Matematica Discreta,UTET, CittaStudiEdizioni, Milano 1997

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A066049, A066436, A091176, A110558.

Programs

Mathematica

Select[Range[2,300,2],PrimeQ[#^2/2-1]&] (* Harvey P. Dale, Apr 05 2014 *)

PARI

isok(n) = !(n % 2) && isprime(n^2/2 - 1); \\ Michel Marcus, Jul 23 2016

Formula

a(n) = 2*A066049(n) = A110558(n)/2. - Ray Chandler, Dec 25 2003

Extensions

Corrected and extended by Ray Chandler, Dec 25 2003