151107(2)

Ruby

(-1)^k (n / k) の和

オンライン整数列大辞典の
A059851（http://oeis.org/A059851/list）
と比較し、答え合わせしてみる。

def s(n)
  (1..n).inject(0){|s, k| s - (n / k) * (-1) ** k}
end

def A059851(n)
  (0..n).map{|i| s(i)}
end
ary = A059851(74)

# OEIS A059851のデータ
ary0 =
[0,1,1,3,2,4,4,6,4,7,7,9,7,9,9,13,10,12,12,14,12,
 16,16,18,14,17,17,21,19,21,21,23,19,23,23,27,24,
 26,26,30,26,28,28,30,28,34,34,36,30,33,33,37,35,
 37,37,41,37,41,41,43,39,41,41,47,42,46,46,48,46,
 50,50,52,46,48,48]
# 一致の確認
p ary == ary0