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A119664
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Sign in term (2p +/- 1) for triangular numbers of the form p * (2p +/- 1), where the two factors are both primes.
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1
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-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
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OFFSET
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1,1
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LINKS
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Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
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EXAMPLE
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a(1) = -1 since 6 = 2*3 and 3 = 2*2 - 1.
a(4) = +1 since 21 = 3*7 and 7 = 2*3 + 1.
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PROG
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(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
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CROSSREFS
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Cf. A068443.
Sequence in context: A269529 A325931 A156734 * A257075 A330034 A010555
Adjacent sequences: A119661 A119662 A119663 * A119665 A119666 A119667
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KEYWORD
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sign,easy
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AUTHOR
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Greg Huber, Jul 28 2006
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EXTENSIONS
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Terms corrected by Charles R Greathouse IV, Jun 13 2013
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STATUS
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approved
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