A066172 Numbers k such that phi(k+1) - phi(k) = -d(k).

5, 7, 33, 135, 231, 273, 435, 465, 824, 6375, 17115, 33704, 86355, 207315, 469395, 603585, 23211885, 37095465, 101216505, 107127111, 124471215, 255080415, 367016055, 1810565025, 4299030465

Offset

1,1

Comments

These are the k at which phi(k) is decreasing at a rate equal to d(k).

Links

Table of n, a(n) for n=1..25.

Example

For k=5, phi(6) - phi(5) = 2 - 4 = -2 = -d(5).

Mathematica

Select[ Range[ 1, 10^6 ], EulerPhi[ # ] - EulerPhi[ # + 1 ] == DivisorSigma[ 0, # ] & ]

Crossrefs

Cf. A000005, A000010.

Sequence in context: A230997 A243019 A007911 * A175667 A341063 A018353

Adjacent sequences: A066169 A066170 A066171 * A066173 A066174 A066175

Keyword

nonn,more

Author

Joseph L. Pe, Dec 14 2001

Extensions

a(17) - a(23) from Harry J. Smith, Feb 04 2010

a(24) from Amiram Eldar, Sep 28 2019

a(25) from Sean A. Irvine, Oct 03 2023

Status

approved