A066950 Numbers k such that phi(sigma(k)) + sigma(phi(k)) = 2k.

1, 11, 18, 75, 172, 372, 482, 732, 3276, 7965, 13068, 46476, 103488, 252240, 727956, 1795605, 6030528, 8769600, 15436092, 56747712, 224518995, 297780084, 317961000, 855489144, 2691137721, 58016937600, 127198874880, 228517311456, 411814778112

Offset: 1

Author

Joseph L. Pe, Jan 25 2002

Keywords

• more
• nonn

Comments

a(26) > 3*10^10. - Donovan Johnson, Mar 05 2014

Examples

			phi(sigma(11)) + sigma(phi(11)) = phi(12) + sigma(10) = 4 + 18 = 2*11, so 11 is in the sequence.
		

Crossrefs

Cf. A000010, A000203.

Programs

Mathematica

f[n_] := EulerPhi[DivisorSigma[1, n]]+DivisorSigma[1, EulerPhi[n]]; Select[Range[1, 10^5], f[ # ]==2*#&]

Extensions

Edited by Dean Hickerson, Jan 26 2002

a(20)-a(25) from Donovan Johnson, Sep 16 2009

a(26)-a(28) from Kevin P. Thompson, Apr 19 2022

a(29) from Kevin P. Thompson, Sep 04 2022