A076648 Numbers k such that phi(k-1) + phi(k+1) = sigma(k)/2.

315, 351, 819, 3375, 24921, 47520, 99540, 107541, 125631, 189175, 410805, 763665, 877365, 1680855, 2480555, 6911079, 7849479, 9646395, 11245365, 12528165, 14242800, 14684055, 16921191, 17194365, 19395025, 27782160, 33830685, 34823075, 36278649, 43955955

Offset

1,1

Links

Table of n, a(n) for n=1..30.

Example

phi(314) + phi(316) = 156 + 156 = 312 = sigma(315)/2, so 315 is a term.

Mathematica

Select[Range[2, 10^5], EulerPhi[ # - 1] + EulerPhi[ # + 1] == (1/2)DivisorSigma[1, # ] &]

Prog

(PARI) is(k) = if(k < 2, 0, eulerphi(k-1) + eulerphi(k+1) == sigma(k)/2); \\ Amiram Eldar, Nov 10 2024

Crossrefs

Cf. A000010, A000203.

Sequence in context: A361034 A210889 A210891 * A323380 A256575 A295990

Adjacent sequences: A076645 A076646 A076647 * A076649 A076650 A076651

Keyword

nonn

Author

Joseph L. Pe, Oct 23 2002

Extensions

a(8)-a(30) from Donovan Johnson, Mar 01 2012

Status

approved