A106549 a(n) = -1 if 2*n-1 is a prime, 1 if 2*n-1 is a prime squared, or 0 otherwise.

0, -1, -1, -1, 1, -1, -1, 0, -1, -1, 0, -1, 1, 0, -1, -1, 0, 0, -1, 0, -1, -1, 0, -1, 1, 0, -1, 0, 0, -1, -1, 0, 0, -1, 0, -1, -1, 0, 0, -1, 0, -1, 0, 0, -1, 0, 0, 0, -1, 0, -1, -1, 0, -1, -1, 0, -1, 0, 0, 0, 1, 0, 0, -1, 0, -1, 0, 0, -1, -1, 0, 0, 0, 0, -1, -1, 0, 0, -1, 0, 0, -1, 0, -1, 1, 0, -1

Offset

1,1

Links

Table of n, a(n) for n=1..87.

Example

a(1)=0 because 1 is neither a prime nor a prime squared;
a(2)=-1 because 3 is a prime;
a(5)=1 because 9 = 3^2 is a prime squared.

Mathematica

Table[Which[PrimeQ[2n+1], -1, PrimeQ[Sqrt[2n+1]], 1, True, 0], {n, 0, 120}] (* Harvey P. Dale, Jul 05 2022 *)

Crossrefs

Sequence in context: A242647 A167393 A275606 * A075897 A135947 A360123

Adjacent sequences: A106546 A106547 A106548 * A106550 A106551 A106552

Keyword

easy,sign

Author

Giovanni Teofilatto, May 09 2005

Extensions

Definition changed to match the data by N. J. A. Sloane, Jul 05 2022 at the suggestion of Harvey P. Dale.

Status

approved