■ある無限級数(その183)

 (その181)−(その182)の補足である.

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[補1]一般に,

  1/1−1/2+1/3−1/4+・・・=log2

の項の順序を,正の項をm個,負の項をn個ずつ交互に並べ替えてできる級数の和は

  log2+1/2・logm/n

となる.

(証明)並べ替えた数列

{(1/1+1/3+1/5+・・・+1/(2m−1))−(1/2+1/4+1/6+・・・+1/2n)}+{(1/(2m+1)+1/(2m+3)+1/(2m+5)+・・・+1/(4m−1))−(1/(2n+2)+1/(2n+4)+1/(2n+6)+・・・+1/4n)}+・・・

のk次部分和は,

{(1/1−1/2+1/3−1/4+1/5+・・・+1/(2km−1)−1/2km)+(1/2+1/4+1/6+・・・+1/2km)−(1/2+1/4+1/6+・・・+1/2kn}

=log2+o(1/km)+1/2(log(km)+logγ+o(1/km))−1/2(log(kn)+logγ+o(1/kn)

=log2+1/2・log(m/n)+o(1/km)+o(1/kn)

}+{(1/(2m+1)+1/(2m+3)+1/(2m+5)+・・・+1/(4m−1))−(1/(2n+2)+1/(2n+4)+1/(2n+6)+・・・+1/4n)}+・・・

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[補2]素数の逆数の和Σ(1/p)については

  lim{Σ(1/p)−loglogn}→0.26149・・・

 Σ(1/p)〜log(logx) (pはp≦xの素数を動く,証明略)

log(logx)は1/(xlogx)の原始関数です.

[1]Σ1/klogk〜∫dx/xlogx〜log(logn)+O(1)

[2]Σ1/(n+k)=Σ1/k−Σ1/k

=(log(2n)+logγ+o(1/2n))−(log(n)+logγ+o(1/n))

=log2+o(1/n)

[3]Σ(n−k+1)/k=−n+1+(n+1)Σ1/k

〜−n+(n+1)(log(n)+logγ+o(1/n))

〜n(logn−1)〜logn!

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