A179882 - OEIS

%I #25 Aug 19 2018 16:41:42

%S 1,7,15,31,39,55,71,111,119,151,175,177,231,239,255,303,311,313,319,

%T 329,335,337,345,375,391,393,479,521,559,575,591,593,601,607,623,655,

%U 657,679,777,785,791,823,855,863,871,879,889,905,911,929,937,959,961,991

%N 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 Subsequence of A179873 and A179874.

%C It appears that for n >= 3, (4*A005384(n)+1)/3 is a subsequence. - _Hilko Koning_, Jul 27 2018

%C 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 Wikipedia, <a href="https://en.wikipedia.org/wiki/Contraharmonic_mean">Contraharmonic mean</a>

%F a(n) = A175505(A179877(n)) / A175506(A179877(n));

%F a(n) = A175505(A179878(n)) / A175506(A179878(n)).

%t {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 (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 Cf. A179871, A179872, A179873, A179874, A179875, A179876, A179877, A179878, A179879, A179880, A179883, A179884, A179885, A179886, A179887, A179890, A179891.

%K nonn

%O 1,2

%A _Jaroslav Krizek_, Jul 30 2010, Jul 31 2010

%E More terms from _Michel Marcus_, Jul 27 2018