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A164646
Numbers n such that sigma(n)/phi(n) = 9/4.
6
51, 477, 595, 3567, 17765, 20735, 41615, 104931, 276651, 470721, 493493, 599169, 834591, 993395, 1092845, 1242505, 1318521, 1479981, 1490645, 1712037, 2344045, 2736305, 2912463, 2986941, 2990709, 3042873, 3187917, 3277611, 3295821, 3767331, 4686039, 5059881
OFFSET
1,1
COMMENTS
A subsequence of A011257.
If 3^{k+1}-1 = d*D such that p = 2*b^{k+1}*(d+1) - 1 and q = 2*(b^{k+1}+D)-1 are distinct primes, then n = 3^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=3; in A068390: b=2, in A164648: b=5).
LINKS
MATHEMATICA
Select[Range[506*10^4], DivisorSigma[1, #]/EulerPhi[#]==9/4&] (* Harvey P. Dale, Jun 22 2019 *)
PROG
(PARI) for( n=1, 1e7, sigma(n)==9/4*eulerphi(n) && print1(n", "))
CROSSREFS
Cf. A000010 (=phi), A000203 (=sigma), A068390 (sigma/phi=4), A163667 (sigma/phi=9), A164647 (sigma/phi=16/9).
Sequence in context: A222910 A259692 A204215 * A128511 A347922 A355418
KEYWORD
nonn
AUTHOR
M. F. Hasler, Aug 22 2009
STATUS
approved