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A063103
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Numbers n such that sigma(usigma(n) is prime.
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2
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OFFSET
| 1,1
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EXAMPLE
| n=8: usigma(8) = 9 and sigma(9) = 13, a prime. n=2667: usigma(2667) = 4096 and sigma(4096) = 8191, a prime.
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MATHEMATICA
| us[n_Integer] := (d = Divisors[n]; l = Length[d]; k = 1; s = n; While[k < l, If[ GCD[ d[[k]], n/d[[k]] ] == 1, s = s + d[[k]]]; k++ ]; s); Do[m = n; If[ PrimeQ[ DivisorSigma[1, us[n]]], Print[n]], {n, 1, 10^7} ]
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PROG
| (PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)); for(n=1, 10^7, if(isprime(sigma(u(n))), print(n)))
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CROSSREFS
| Cf. A034448, A063508.
Sequence in context: A081466 A092592 A162185 * A058847 A088110 A122759
Adjacent sequences: A063100 A063101 A063102 * A063104 A063105 A063106
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KEYWORD
| nonn
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AUTHOR
| Jason Earls (zevi_35711(AT)yahoo.com), Aug 07 2001
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