A119664 Sign in term (2p +/- 1) for triangular numbers of the form p * (2p +/- 1), where the two factors are both primes.

-1, 1, -1, 1, 1, -1, 1, -1, 1, 1, -1, -1, 1, 1, -1, 1, 1, -1, 1, 1, -1, -1, 1, 1, 1, -1, -1, -1, 1, 1, 1, -1, 1, 1, -1, -1, -1, 1, -1, -1, 1, 1, -1, 1, 1, -1, 1, -1, -1, 1, -1, -1, -1, 1, 1, 1, -1, 1, -1, 1, -1, 1, 1, 1, -1, -1, -1, 1, -1, 1, -1, -1, -1, 1, 1, 1, 1, -1

Offset

1,1

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Example

a(1) = -1 since 6 = 2*3 and 3 = 2*2 - 1.
a(4) = +1 since 21 = 3*7 and 7 = 2*3 + 1.

Prog

(PARI) list(x)=my(v=List()); forprime(p=2, (sqrtint(x\1*8+1)+1)\4, if(isprime(2*p-1), listput(v, -1)); if(isprime(2*p+1), listput(v, 1))); Vec(v) \\ Charles R Greathouse IV, Jun 13 2013

Crossrefs

Cf. A068443.

Sequence in context: A269529 A325931 A156734 * A257075 A330034 A010555

Adjacent sequences: A119661 A119662 A119663 * A119665 A119666 A119667

Keyword

sign,easy

Author

Greg Huber, Jul 28 2006

Extensions

Terms corrected by Charles R Greathouse IV, Jun 13 2013

Status

approved