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A076543 Sum(k=1 to n) k*sqf(k); sqf(k)=1 if k is squarefree and sqf(k)=-1 otherwise. 1

%I

%S 1,3,6,2,7,13,20,12,3,13,24,12,25,39,54,38,55,37,56,36,57,79,102,78,

%T 53,79,52,24,53,83,114,82,115,149,184,148,185,223,262,222,263,305,348,

%U 304,259,305,352,304,255,205,256,204,257,203,258,202,259,317,376,316,377

%N Sum(k=1 to n) k*sqf(k); sqf(k)=1 if k is squarefree and sqf(k)=-1 otherwise.

%C Surprisingly, first 12 terms are also in A074170.

%H Harvey P. Dale, <a href="/A076543/b076543.txt">Table of n, a(n) for n = 1..1000</a>

%t Accumulate[Table[If[SquareFreeQ[n],n,-n],{n,70}]] (* _Harvey P. Dale_, Mar 18 2015 *)

%o (PARI) a(n) = sum(k = 1, n, if (issquarefree(k), k, -k)); \\ _Michel Marcus_, Oct 02 2013

%Y Cf. A074170.

%K easy,nonn

%O 1,2

%A _Zak Seidov_, Oct 19 2002

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Last modified November 16 00:11 EST 2018. Contains 317252 sequences. (Running on oeis4.)