A070810 Nonprime numbers k such that phi(k-phi(k)) = A054571(k) is a power of 2.

4, 6, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 25, 28, 32, 36, 38, 39, 40, 42, 44, 46, 48, 50, 55, 56, 58, 62, 64, 65, 72, 76, 77, 80, 84, 88, 92, 94, 96, 100, 112, 116, 118, 124, 128, 134, 144, 152, 158, 160, 165, 168, 176, 184, 188, 192, 200, 202, 224, 232, 235, 236

Offset: 1

Author

Labos Elemer, May 08 2002

Keywords

• nonn

Examples

			For k = 168: 168 - phi(168) = 168-48 = 120, phi(120) = 32, a power of 2.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..6018 (terms below 10^10)

Crossrefs

Cf. A000010, A054571, A051953, A070556, A070806, A070807, A070809, A070811, A070812.

Programs

Mathematica

Do[s=EulerPhi[n-EulerPhi[n]]; If[IntegerQ[Log[2, s]]&&!PrimeQ[n], Print[n]], {n, 1, 256}]

PARI

is(k) = if(k == 1 || isprime(k), 0, my(m = eulerphi(k - eulerphi(k))); m >> valuation(m, 2) == 1); \\ Amiram Eldar, Nov 08 2024