■約数関数のおおまかな上界と下界(その4)

 σ3(n)をnの約数の3乗和を表すとする.

  σ3(15)=1^3+3^3+5^3+15^3=3528

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[1]σ(1)σ(2n−1)+σ(3)σ(2n−3)+σ(5)σ(2n−5)+・・・+σ(2n−1)σ(1)=σ3(n)

  S8(2n)=S4(4)S4(8n−4)+S4(12)S4(8n−12)+・・・+S4(8n−4)S4(4)

  S4(4(2n−1))=σ(2n−1)

より,S8(2n)=σ3(n)

[2]nが2のベキ乗のとき

n       1 2  4   8   16 2^n

S8(8n)   1 8 64 512 4096 n^3

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