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Search: id:A164650
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| A164650 |
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Numbers n such that sigma(n)/phi(n) = 49/36. |
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+0
2
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| 679, 10127, 20273, 672203, 971261, 1133639, 1247129, 1336231, 1646743, 1701089, 2369471, 2674969, 2722499, 2989909, 3160079, 3597659, 4545749, 6333503, 7127861, 9357101
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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A subsequence of A011257.
If 7^{k+1}-1 = d*D such that p = 2*7^{k+1}*(d+1)-1 and q = 2*(7^{k+1}+D)-1 are distinct primes, then n = 7^k*p*q is a term of this sequence.
The same theorem holds for sequences of numbers such that sigma/phi=b^2/(b-1)^2 with other primes b (here b=7), cf. A068390, A164646, A164648.
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PROGRAM
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(PARI) for( n=1, 1e7, sigma(n)==49/36*eulerphi(n) && print1(n", "))
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CROSSREFS
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Cf. A000010 (=phi), A000203 (=sigma), A068390 (sigma/phi=4), A163667 (sigma/phi=9), A164646-A164649.
Sequence in context: A097771 A121105 A046514 this_sequence A097772 A154036 A118509
Adjacent sequences: A164647 A164648 A164649 this_sequence A164651 A164652 A164653
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KEYWORD
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more,nonn
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AUTHOR
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M. F. Hasler (mhasler(AT)univ-ag.fr), Aug 22 2009
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