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Hermite's identity
・ Hermite's problem
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Hermite's identity : ウィキペディア英語版
Hermite's identity

In mathematics, Hermite's identity, named after Charles Hermite, gives the value of a summation involving the floor function. It states that for every real number ''x'' and for every positive integer ''n'' the following identity holds:〔.〕〔.〕
: \sum_^\left\lfloor x+\frac\right\rfloor=\lfloor nx\rfloor .
==Proof==
Split x into its integer part and fractional part, x=\lfloor x\rfloor+\. There is exactly one k'\in\ with
:\lfloor x\rfloor=\left\lfloor x+\frac\right\rfloor\le x<\left\lfloor x+\frac\right\rfloor=\lfloor x\rfloor+1.
By subtracting the same integer \lfloor x\rfloor from inside the floor operations on the left and right sides of this inequality, it may be rewritten as
:0=\left\lfloor \+\frac\right\rfloor\le \<\left\lfloor \+\frac\right\rfloor=1.
Therefore,
:1-\frac\le \<1-\frac ,
and multiplying both sides by n gives
:n-k'\le n\, \
Now if the summation from Hermite's identity is split into two parts at index k', it becomes
: \sum_^\left\lfloor x+\frac\right\rfloor
=\sum_^ \lfloor x\rfloor+\sum_^ (\lfloor x\rfloor+1)=n\, \lfloor x\rfloor+n-k'
=n\, \lfloor x\rfloor+\lfloor n\,\\rfloor=\left\lfloor n\, \lfloor x\rfloor+n\, \ \right\rfloor=\lfloor nx\rfloor.


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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