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%I #9 Oct 29 2023 22:05:39
%S -1,2,-8,11,-15,15,-35,48,-43,35,-87,103,-67,83,-177,162,-128,145,
%T -217,277,-223,143,-387,443,-208,302,-518,432,-410,370,-592,771,-449,
%U 421,-879,850,-520,566,-1134,1024,-658,842,-1008,1310,-1056,534,-1676,1714,-737
%N a(n) = Sum_{k=1..n} (-1)^k*k^2*floor(n/k).
%F a(n) = 8*A064602(floor(n/2))-A064602(n).
%t a[n_]:=Sum[ (-1)^k*k^2*Floor[n/k],{k,n}]; Array[a,49] (* _Stefano Spezia_, Oct 29 2023 *)
%o (Python)
%o from math import isqrt
%o def A366915(n): return (-(t:=isqrt(m:=n>>1))**2*(t+1)*((t<<1)+1)+sum((q:=m//k)*(6*k**2+q*((q<<1)+3)+1) for k in range(1,t+1))<<2)//3+((s:=isqrt(n))**2*(s+1)*((s<<1)+1)-sum((q:=n//k)*(6*k**2+q*((q<<1)+3)+1) for k in range(1,s+1)))//6
%o (PARI) a(n) = sum(k=1, n, (-1)^k*k^2*(n\k)); \\ _Michel Marcus_, Oct 29 2023
%Y Cf. A024919, A064602.
%K sign
%O 1,2
%A _Chai Wah Wu_, Oct 28 2023
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