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

Author

Artur Jasinski, Jan 23 2008

Keywords

• nonn
• easy

Comments

Numbers that occur in A002618 but not in A036689.

Links

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

Crossrefs

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

Programs

Mathematica

a = {}; Do[If[!PrimeQ[n], AppendTo[a, n EulerPhi[n]]], {n, 1, 100}]; a

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