A089693 Numbers n such that phi(n) = 2^bigomega(n).

1, 3, 10, 20, 30, 40, 60, 80, 120, 160, 240, 320, 480, 640, 960, 1280, 1920, 2560, 3840, 5120, 7680, 10240, 15360, 20480, 30720, 40960, 61440, 81920, 122880, 163840, 245760, 327680, 491520, 655360, 983040

Offset: 1

Author

Benoit Cloitre, Jan 06 2004

Keywords

• nonn
• easy

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000
• Index entries for linear recurrences with constant coefficients, signature (0,2)

Crossrefs

Cf. A000010, A061142, A001222, A070004.

Programs

Mathematica

Select[Range[100000],EulerPhi[#]==2^PrimeOmega[#]&] (* or *) Join[{1,3,10},LinearRecurrence[{0,2},{20,30},20]] (* Harvey P. Dale, Mar 01 2012 *)

Formula

a(n) = 2*a(n-2) for n>5. [Harvey P. Dale, Mar 01 2012]

For n > 2, A001222(a(n)) = 1 + floor(n/2). - Enrique Pérez Herrero, Mar 28 2012

For n > 1, a(2n) = 5*2^n and a(2n+1) = 15*2^(n-1). - Enrique Pérez Herrero, Mar 28 2012

Extensions

More terms from Harvey P. Dale, Mar 01 2012