A280619 Integers m such that sigma(m) - eulerphi(m) <= 4*sqrt(m).

1, 2, 3, 4, 5, 7, 8, 9, 11, 13, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 121, 127, 131, 137, 139, 149, 151, 157, 163, 167, 169, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241

Offset: 1

Author

Michel Marcus, Jan 06 2017

Keywords

• nonn
• easy

Comments

Equals {1, 8} union A000430 (primes and square of primes).

Equals 8 union A166684.

Links

• Indranil Ghosh, Table of n, a(n) for n = 1..10000
• Mathematical Reflections, Solution to Problem O261, Issue 3, 2013, p. 24.

Crossrefs

A000430 is the main entry for this sequence.

Cf. A000010, A000203, A166684.

Programs

Mathematica

Select[Range[250],DivisorSigma[1,#]-EulerPhi[#]<=4Sqrt[#]&] (* Harvey P. Dale, Jul 10 2018 *)

PARI

isok(n) = sigma(n) - eulerphi(n) <= 4*sqrt(n); \\ Michel Marcus, Jan 06 2017