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A103176
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Let p = prime(sigma(n)) and q = prime(phi(n)), then p is in the sequence if p-q = 6.
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1
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13, 19, 43, 113, 463, 619, 863, 1789, 2273, 2383, 4519, 4789, 4937, 5443, 5507, 5653, 8237, 10459, 13007, 13697, 16063, 16453, 17389, 18313, 18919, 20903, 21193, 21319, 21383, 23567, 24109, 25309, 26267, 27947, 28283, 29573, 30559, 31183, 31517
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OFFSET
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1,1
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COMMENTS
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Conjecture: In all cases sigma(n)-phi(n)=2, i.e., n is prime.
Proof: Suppose n is composite. Then sigma(n) > n + sqrt(n) and phi(n) <= n - sqrt(n) and so prime(sigma(n)) - prime(phi(n)) >= sigma(n) - phi(n) > 2*sqrt(n) > 6 for n > 9. - Charles R Greathouse IV, May 15 2013
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LINKS
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EXAMPLE
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n=3719, sigma(n)=3720, phi(n)=3718, a(n)=p(sigma(n))=34847.
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MATHEMATICA
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Do[g=n; a=Prime[u=DivisorSigma[1, n]]; b=Prime[w=EulerPhi[n]]; s=a-b; If[Equal[s, 6], Print[{n, a, b, u, w, u-w}]; ta=Append[ta, a]], {n, 1, 10000}] ta=Delete[ta, 1]
Prime[DivisorSigma[1, #]]&/@Select[Range[5000], Prime[DivisorSigma[ 1, #]] == Prime[ EulerPhi[#]]+6&] (* Harvey P. Dale, Sep 22 2016 *)
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PROG
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(PARI) p=2; q=3; forprime(r=5, 1e6, if(r-p==6 && isprime(primepi(q)), print1(r", ")); p=q; q=r) \\ Charles R Greathouse IV, May 15 2013
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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