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A248793
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Sigma(n) - 1 for n such that sigma(n) - 1 is prime.
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2
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2, 3, 5, 11, 7, 17, 11, 13, 23, 23, 17, 19, 41, 31, 23, 59, 41, 29, 71, 31, 47, 53, 47, 37, 59, 89, 41, 43, 83, 71, 47, 71, 97, 53, 71, 79, 89, 59, 167, 61, 103, 83, 67, 71, 73, 113, 139, 167, 79, 83, 223, 107, 131, 179, 89, 233, 167, 127, 251, 97, 101, 103
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OFFSET
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1,1
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COMMENTS
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a(n) = corresponding values of primes p = sigma(A248792(n)) - 1, where A248792(n) = numbers n such that sigma(n) - 1 is prime.
If there are at least two numbers k, h such that a(k) = a(h) = p, then p is in A158913.
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LINKS
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FORMULA
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MAPLE
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F:= proc(n)
local r;
r:= numtheory:-sigma(n)-1;
if isprime(r) then r else NULL fi
end proc:
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MATHEMATICA
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a248793[n_Integer] :=
Cases[DivisorSigma[1, #] - 1 & /@ Range[n], _?PrimeQ]; a248793[104] (* Michael De Vlieger, Nov 07 2014 *)
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PROG
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(Magma) [a: n in [1..1000] | IsPrime(a) where a is SumOfDivisors(n)-1]
(PARI) for(n=1, 10^3, if(isprime(sigma(n)-1), print1(sigma(n)-1, ", "))) \\ Derek Orr, Nov 01 2014
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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