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A065873
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Usigma(n) +/-1 are twin primes.
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1
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3, 5, 6, 10, 11, 17, 18, 20, 26, 29, 30, 38, 41, 44, 45, 46, 51, 55, 56, 59, 71, 80, 85, 88, 90, 98, 101, 105, 107, 114, 116, 118, 126, 132, 137, 140, 141, 145, 149, 150, 152, 153, 155, 158, 160, 161, 177, 178, 179, 185, 188, 191, 197, 203, 206, 207, 209, 212, 227
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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LINKS
| Harry J. Smith, Table of n, a(n) for n=1,...,1000
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MATHEMATICA
| f[n_] := Block[ {a = FactorInteger[n], k = l = s = 1}, l = Length[a]; While[k <= l, s = s * (a[[k, 1]]^a[[k, 2]] + 1); k++ ]; Return[s]]; Select[ Range[250], PrimeQ[ f[ # ] + 1] && PrimeQ[ f[ # ] - 1] & ]
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PROG
| (PARI) usigma(n)= { local(f, s=1); f=factor(n); for(i=1, matsize(f)[1], s*=1 + f[i, 1]^f[i, 2]); return(s) } { n=0; for (m=1, 10^9, u=usigma(m); if (isprime(u - 1) && isprime(u + 1), write("b065873.txt", n++, " ", m); if (n==1000, return)) ) } [From Harry J. Smith (hjsmithh(AT)sbcglobal.net), Nov 02 2009]
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CROSSREFS
| Cf. A034448.
Sequence in context: A175304 A140951 A190243 * A072282 A047327 A028927
Adjacent sequences: A065870 A065871 A065872 * A065874 A065875 A065876
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KEYWORD
| nonn
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AUTHOR
| Robert G. Wilson v (rgwv(AT)rgwv.com), Dec 07 2001
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