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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, -1 (list; graph; refs; listen; history; text; internal format)
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

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Last modified July 6 11:37 EDT 2022. Contains 355110 sequences. (Running on oeis4.)