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A068017
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Composite n such that -1+Sigma[n]and 1+Sigma[n] are twin primes.
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2
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6, 10, 20, 24, 26, 30, 38, 46, 51, 55, 85, 88, 105, 114, 118, 126, 135, 136, 141, 145, 147, 155, 158, 161, 177, 178, 185, 203, 206, 207, 209, 216, 230, 236, 238, 255, 278, 296, 321, 344, 346, 355, 371, 377, 384, 391, 396, 398, 416, 424, 447, 462, 486, 500
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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EXAMPLE
| For n=46, Sigma[46]=1+2+23+46=72, for n=51, Sigma[51]=1+3+17+51=72 and also for n=55, Sigma[55]=1+5+11+55=72 is the middle term of {71,73} twins.
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MATHEMATICA
| Do[s=-1+DivisorSigma[1, n]; s1=1+DivisorSigma[1, n]; If[PrimeQ[s]&&PrimeQ[s1]&&!PrimeQ[n], Print[n]], {n, 1, 2000}]
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CROSSREFS
| Cf. A000203.
Sequence in context: A095985 A145351 A015783 * A169810 A095146 A163172
Adjacent sequences: A068014 A068015 A068016 * A068018 A068019 A068020
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu), Feb 08 2002
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