A240882 Numbers m such that m - 4*k^2 is a prime for all k > 0 with k^2 < m/4.

6, 7, 9, 11, 15, 21, 23, 27, 33, 35, 47, 77, 83, 143, 167, 227, 437

Offset

1,1

Comments

No other terms with m < 1000000. - Colin Barker, Apr 14 2014

If it exists, a(18) > 10^9. - Jon E. Schoenfield, Mar 17 2024

Links

Table of n, a(n) for n=1..17.

Example

21 is in this sequence because 21 - 4*1^2 = 17 and 21 - 4*2^2 = 5 are both prime.

Mathematica

n=6; Monitor[Parallelize[While[True, If[MemberQ[PrimeQ[Table[n-4*k^2, {k, 1, Floor[Sqrt[n/4]]}]], False]==False, Print[n]]; n++]; n], n] (* J.W.L. (Jan) Eerland, Mar 17 2024 *)

Prog

(PARI) isOK(n) = k=1; until(k^2>=n/4, if(!isprime(n-4*k^2), return(0)); k++); 1;
for(n=3, 1000000, if(isOK(n), print1(n, ", "))) \\ Colin Barker, Apr 14 2014

Crossrefs

Cf. A240842.

Sequence in context: A175221 A094010 A100348 * A241266 A095908 A094698

Adjacent sequences: A240879 A240880 A240881 * A240883 A240884 A240885

Keyword

nonn,more

Author

Ilya Lopatin and Juri-Stepan Gerasimov, Apr 14 2014

Extensions

One missing term and one additional term from Colin Barker, Apr 14 2014

Status

approved