A137148 a(n) = k*phi(k), where k is the n-th nonprime number.

1, 8, 12, 32, 54, 40, 48, 84, 120, 128, 108, 160, 252, 220, 192, 500, 312, 486, 336, 240, 512, 660, 544, 840, 432, 684, 936, 640, 504, 880, 1080, 1012, 768, 2058, 1000, 1632, 1248, 972, 2200, 1344, 2052, 1624, 960, 1860, 2268, 2048, 3120, 1320, 2176, 3036

Offset

1,2

Comments

Numbers that occur in A002618 but not in A036689.

Links

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

Formula

a(n) = A002618(A018252(n)). - R. J. Mathar, Jan 18 2021

Sum_{n>=1} 1/a(n) = A065484 - A136141 = 1.430699927388... . - Amiram Eldar, Oct 26 2024

Mathematica

a = {}; Do[If[!PrimeQ[n], AppendTo[a, n EulerPhi[n]]], {n, 1, 100}]; a
# EulerPhi[#]&/@Select[Range[100], !PrimeQ[#]&] (* Harvey P. Dale, May 23 2026 *)

Crossrefs

Cf. A002618, A018252, A036689, A065484, A136141.

Sequence in context: A083485 A217156 A066934 * A211778 A385713 A045018

Adjacent sequences: A137145 A137146 A137147 * A137149 A137150 A137151

Keyword

nonn,easy

Author

Artur Jasinski, Jan 23 2008

Status

approved