A083243 Numbers k for which there are more divisors and coprimes than other numbers less than k: A045763(k) < A073757(k) or A045763(k) < k/2 or A073757(k) > k/2.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 49, 51, 52, 53, 55, 56, 57, 58, 59, 61, 62, 63, 64, 65, 67, 68, 69, 71, 73, 74, 75, 76

Offset

1,2

Links

Table of n, a(n) for n=1..68.

Formula

{ k : d(k) + phi(k) - 1 > k/2 }.

Example

30 is not in the sequence because d(30) + phi(30) - 1 = 8 + 8 - 1 = 15. There are as many divisors and coprimes as there are numbers j <= 30 that neither divide nor are coprime to 30.
50 is not here because d(50) + phi(50) - 1 = 6 + 20 - 1 = 25. There are as many divisors and coprimes as there are numbers j < 50 that neither divide nor are coprime to 50.
146 is here because d(146) + phi(146) - 1 = 4 + 72 - 1 = 75; 146/2 = 73, and 75 > 73.
61455 is here because d(61455) + phi(61455) - 1 = 16 + 30720 - 1 = 30735; 61455/2 = 30727 + 1/2, and 30735 > 61455/2.

Mathematica

Do[r=EulerPhi[n]; d=DivisorSigma[0, n]; u=n-r-d+1; If[Greater[n-u, n/2], Print[n, {d, r, u}]], {n, 1, 100}]
(* Second program: *)
Select[Range[120], DivisorSigma[0, #] + EulerPhi[#] - 1 > #/2 &] (* Michael De Vlieger, Aug 22 2023 *)

Crossrefs

Cf. A000005, A000010, A045763, A073757, A020488, A083244, A083245.

Sequence in context: A273888 A192218 A070915 * A004441 A004438 A109425

Adjacent sequences: A083240 A083241 A083242 * A083244 A083245 A083246

Keyword

nonn

Author

Labos Elemer, May 07 2003

Extensions

Data corrected and entry edited by Michael De Vlieger, Aug 22 2023

Status

approved