A076647 Numbers n such that phi(n) + phi(n+1) = sigma(n)/2.

56, 456, 476, 828, 846, 1196, 2060, 2780, 4484, 7530, 9174, 11420, 11475, 11865, 13454, 22154, 34916, 35385, 52185, 67308, 105798, 140895, 168872, 190840, 219852, 241892, 244074, 267225, 303834, 317564, 330484, 335440, 460292, 580970, 658515

Offset: 1

Author

Joseph L. Pe, Oct 23 2002

Keywords

• nonn

Examples

			phi(56) + phi(57) = 24 + 36 = 60 = 1/2 sigma(56), so 56 is a term of the sequence.
		

Links

• Donovan Johnson, Table of n, a(n) for n = 1..200

Crossrefs

Cf. A092404.

Programs

Maple

with(numtheory): p:=proc(n) if phi(n)+phi(n+1)=sigma(n)/2 then n else fi end: seq(p(n),n=1..2500000); # Emeric Deutsch, Sep 02 2005

Mathematica

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

PARI

isok(n) = eulerphi(n) + eulerphi(n+1) == sigma(n)/2; \\ Michel Marcus, Aug 15 2019

Extensions

More terms from Emeric Deutsch, Sep 02 2005