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A154585 a(n)=abs{(-1)^k*Sum{k=1..n}{[(n-k+1) mod k]}} 3

%I

%S 0,1,1,0,1,2,4,0,3,4,5,4,1,10,4,3,1,2,9,2,11,12,17,11,0,13,0,1,6,7,23,

%T 8,7,20,10,9,8,25,14,13,4,3,20,13,34,35,34,26,8,13,6,5,8,25,24,1,26,

%U 27,34,33,4,37,25,6,11,12,11,16,37,38,60,59,24,25,0,19,40,41,54,14,25,26,51

%N a(n)=abs{(-1)^k*Sum{k=1..n}{[(n-k+1) mod k]}}

%F a(n)=abs{(-1)^k*A004125}

%e a(5)=-(5 mod 1)+(4 mod 2)-(3 mod 3)+(2 mod 4)-(1 mod 5)=-0+0-0+2-1=1

%p P:=proc(i) local a,n; for n from 1 by 1 to i do a:=abs(sum('(-1)^k*((n-k+1) mod k)','k'=1..n)); print(a); od; end: P(100);

%Y A004125, A154586

%K easy,nonn

%O 1,6

%A _Paolo P. Lava_ and _Giorgio Balzarotti_, Jan 12 2009

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Last modified September 16 07:19 EDT 2021. Contains 347469 sequences. (Running on oeis4.)