151210

Ruby


Number of ways to express 1 as the sum of unit fractions such that the sum of the denominators is n(2)

第100項まで求めてみた。

# 和因子はmin以上max以下
def partition(n, min, max)
  return [[]] if n == 0
  [max, n].min.downto(min).flat_map{|i| partition(n - i, min, i).map{|rest| [i, *rest]}}
end

def A051908(n)
  # 1 = 1, 1 = 1 / 1の一通り
  ary = [1]
  (2..n).each{|m|
    cnt = 0
    partition(m, 2, m).each{|ary|
      cnt += 1 if ary.inject(0){|s, i| s + 1 / i.to_r} == 1
    }
    ary << cnt
  }
  ary
end

p A051908(10 ** 2)

出力結果
[1, 0, 0, 1, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 3, 0, 1, 1, 1, 1, 2, 3, 2, 2, 1, 2, 2, 2, 4, 5, 5, 2, 4, 5, 5, 9, 4, 4, 6, 4, 4, 7, 8, 4, 10, 9, 9, 11, 8, 13, 13, 15, 16, 21, 18, 16, 22, 19, 18, 30, 24, 19, 26, 28, 26, 29, 35, 29, 44, 28, 47, 48, 43, 44, 59, 49, 51, 72, 65, 64, 79, 70, 78, 87, 98, 83, 110, 103, 96, 126, 101, 134, 137]