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A179882 a(n) is the corresponding value of contraharmonic mean B(h) of numbers k such that gcd(k, h) = 1 (k < h) for numbers h from A179877(n) and A179878(n).

%I A179882 #41 Jun 24 2025 19:23:52
%S A179882 1,7,15,31,39,55,71,111,119,151,175,177,231,239,255,303,311,313,319,
%T A179882 329,335,337,345,375,391,393,479,521,559,575,591,593,601,607,623,655,
%U A179882 657,679,777,785,791,823,855,863,871,879,889,905,911,929,937,959,961,991
%N A179882 a(n) is the corresponding value of contraharmonic mean B(h) of numbers k such that gcd(k, h) = 1 (k < h) for numbers h from A179877(n) and A179878(n).
%C A179882 Subsequence of A179873 and A179874.
%C A179882 It appears that for n >= 3, (4*A005384(n)+1)/3 is a subsequence. - _Hilko Koning_, Jul 27 2018
%C A179882 This happens for this subsequence of A179877: 10, 22, 46, 58, 82, 106, 166, 178, ... apparently "Semiprimes of form prime - 1" >= 10 (see A077065). - _Michel Marcus_, Jul 27 2018
%H A179882 Amiram Eldar, <a href="/A179882/b179882.txt">Table of n, a(n) for n = 1..10000</a>
%H A179882 Wikipedia, <a href="https://en.wikipedia.org/wiki/Contraharmonic_mean">Contraharmonic mean</a>.
%F A179882 a(n) = A175505(A179877(n)) / A175506(A179877(n)).
%F A179882 a(n) = A175505(A179878(n)) / A175506(A179878(n)).
%t A179882 {1}~Join~Select[Partition[Table[ContraharmonicMean@ Select[Range[n - 1], GCD[#, n] == 1 &], {n, 2, 1500}], 2, 1], And[IntegerQ@ First@ #, SameQ @@ #] &][[All, 1]] (* _Michael De Vlieger_, Jul 30 2018 *)
%o A179882 (PARI) lista(nn) = {vch = vector(nn, k, ch(k)); for (i=1, nn-1, if ((vch[i] == vch[i+1]) && !frac(vch[i]), print1(vch[i], ", ")););} \\ _Michel Marcus_, Jul 27 2018
%Y A179882 Cf. A179871, A179872, A179873, A179874, A179875, A179876, A179877, A179878, A179879, A179880, A179883, A179884, A179885, A179886, A179887, A179890, A179891.
%K A179882 nonn
%O A179882 1,2
%A A179882 _Jaroslav Krizek_, Jul 30 2010, Jul 31 2010
%E A179882 More terms from _Michel Marcus_, Jul 27 2018