A056853 Numbers n satisfying phi(n+1) - phi(n-1) = 2.

4, 6, 7, 12, 13, 15, 18, 19, 21, 30, 42, 45, 60, 63, 72, 93, 102, 108, 117, 138, 150, 165, 180, 192, 198, 213, 228, 240, 255, 270, 282, 312, 333, 348, 357, 420, 432, 453, 462, 522, 525, 570, 600, 618, 642, 660, 693, 717, 765, 810, 822, 828, 858, 882, 933, 957

Offset: 1

Author

Joseph L. Pe, Feb 12 2002

Keywords

• nonn

Examples

			phi(13+1)-phi(13-1) = 2, so 13 is a term of the sequence.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010. Essentially the same as A001838.

Programs

Mathematica

Select[Range[10^3], EulerPhi[ # + 1] - EulerPhi[ # - 1] == 2 &]
Flatten[Position[Partition[EulerPhi[Range[1000]],3,1],?(Last[#]-First[#] == 2&),{1},Heads->False]]+1 (* _Harvey P. Dale, Jul 14 2014 *)