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A075997
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a(n) = [n/2] - [n/3] + [n/4] - [n/5] + [n/6] - ..., where [n/k] = floor(n/k).
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4
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0, 0, 1, 0, 2, 1, 2, 1, 4, 2, 3, 2, 5, 4, 5, 2, 6, 5, 6, 5, 8, 5, 6, 5, 10, 8, 9, 6, 9, 8, 9, 8, 13, 10, 11, 8, 12, 11, 12, 9, 14, 13, 14, 13, 16, 11, 12, 11, 18, 16, 17, 14, 17, 16, 17, 14, 19, 16, 17, 16, 21, 20, 21, 16, 22, 19, 20, 19, 22, 19, 20, 19, 26, 25, 26, 21, 24, 21, 22, 21
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OFFSET
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0,5
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COMMENTS
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a(n) is the number of terms among {floor(n/k)}, 1<=k<=n, which are even. - Leroy Quet, Jan 19 2006
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LINKS
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FORMULA
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a(n) = n - Sum_{n/2<k<=n} d(k) + Sum_{1<=k<=n/2} d(k), where d(k) = A000005(k). - Leroy Quet, Jan 19 2006
a(n) = Sum_{i=1..n} floor(n/(2*i)) - floor((n-i)/(2*i)). - Wesley Ivan Hurt, Jan 30 2016
Conjecture: Let f(a,b)=1, if (a+b) mod |a-b| != (a mod |a-b|)+(b mod |a-b|), and 0 otherwise. a(n) = Sum_{k=1..n-1} f(n+k,n-k). - Benedict W. J. Irwin, Sep 23 2016
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EXAMPLE
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a(6) = [6/2]-[6/3]+[6/4]-[6/5]+[6/6]-[6/7]+... = 3-2+1-1+1-0+... = 2.
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MAPLE
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MATHEMATICA
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Table[Sum[Floor[n/(2 i)] - Floor[(n - i)/(2 i)], {i, n}], {n, 0, 100}] (* Wesley Ivan Hurt, Jan 30 2016 *)
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PROG
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(PARI) a(n) = sum(k=2, n, (-1)^k*(n\k)); \\ Michel Marcus, Dec 20 2020
(Python)
from math import isqrt
def A075997(n): return n+(s:=isqrt(n))**2-((t:=isqrt(m:=n>>1))**2<<1)-(sum(n//k for k in range(1, s+1))-(sum(m//k for k in range(1, t+1))<<1)<<1) # Chai Wah Wu, Oct 23 2023
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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