■収束するやせざるや(その5)

【1】条件収束する級数(続き)

  (1−1)+(1/2−1/2)+(1/3−1/3)+(1/4−1/4)+・・・=0ですが,項の順序を並べ替えてできる次の級数の和は?

[Q1](1+1/2−1)+(1/3+1/4−1/2)+(1/5+1/6−1/3)+・・・

[Q2](1+1/2+1/3−1)+(1/4+1/5+1/6−1/2)+(1/7+1/8+1/9−1/3)+・・・

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[A1]

  Σ(1/(2k−1)+1/2k−1/k)

 =Σ(1/(2k−1)−1/2k)

 =1/1−1/2+1/3−1/4+・・・=log2

[A2]logγ=lim{Σ1/k−logn}

  lim{(1+1/2+1/3+・・・+1/3k)−(1+1/2+1/3+・・・+1/k)}

 =lim{(log3k+logγ+o(1/k))−(logk+logγ+o(1/k))}

 =log3

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