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).

1, 7, 15, 31, 39, 55, 71, 111, 119, 151, 175, 177, 231, 239, 255, 303, 311, 313, 319, 329, 335, 337, 345, 375, 391, 393, 479, 521, 559, 575, 591, 593, 601, 607, 623, 655, 657, 679, 777, 785, 791, 823, 855, 863, 871, 879, 889, 905, 911, 929, 937, 959, 961, 991

Offset

1,2

Comments

Subsequence of A179873 and A179874.

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

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

Links

Table of n, a(n) for n=1..54.

Wikipedia, Contraharmonic mean

Formula

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

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

Mathematica

{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 *)

Prog

(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

Crossrefs

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

Sequence in context: A210675 A293361 A117747 * A317510 A137196 A120094

Adjacent sequences: A179879 A179880 A179881 * A179883 A179884 A179885

Keyword

nonn

Author

Jaroslav Krizek, Jul 30 2010, Jul 31 2010

Extensions

More terms from Michel Marcus, Jul 27 2018

Status

approved