A076543 a(n) = Sum_{k=1..n} k*sqf(k) where sqf(k)=1 if k is squarefree and sqf(k)=-1 otherwise.

1, 3, 6, 2, 7, 13, 20, 12, 3, 13, 24, 12, 25, 39, 54, 38, 55, 37, 56, 36, 57, 79, 102, 78, 53, 79, 52, 24, 53, 83, 114, 82, 115, 149, 184, 148, 185, 223, 262, 222, 263, 305, 348, 304, 259, 305, 352, 304, 255, 205, 256, 204, 257, 203, 258, 202, 259, 317, 376, 316, 377

Offset: 1

Author

Zak Seidov, Oct 19 2002

Keywords

• easy
• nonn

Comments

Surprisingly, first 12 terms are also in A074170.

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A074170.

Programs

Mathematica

Accumulate[Table[If[SquareFreeQ[n],n,-n],{n,70}]] (* Harvey P. Dale, Mar 18 2015 *)

PARI

a(n) = sum(k = 1, n, if (issquarefree(k), k, -k)); \\ Michel Marcus, Oct 02 2013