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$\begingroup$

$$1 (N) + 2(N-1) + 3(N -2) + \cdots + i(N - i + 1) + \cdots + N (1) $$

I need to write it in simplest form?

here 1(N) means 1 multiply by N

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    Sorry, but does $N$ stand for a complex number?2012-11-29
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    Babak, $N$ must be a natural number; $i$ is intended as an index, not the square root of $-1$. So the sum is $\sum_{k=0}^{N-1} (1+k)(N-k)$. (Ah, now I see why you asked that. Grijesh, the tags you've chosen are completely inappropriate.)2012-11-29
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    I am tiring to answer some other problem related to computer scince2012-11-29
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    Apart from [the post](http://math.stackexchange.com/questions/1113556/how-to-show-that-sum-k-1n-kn1-k-binomn23) already mentioned by Normal Human, some other posts which are [linked there](http://math.stackexchange.com/questions/linked/1113556) might be interesting for you, too.2015-12-16

3 Answers 3

7

$$ \sum_{k=1}^Nk(N-k+1)=\sum_{k=1}^N kN-\sum_{k=1}^Nk^2+\sum_{k=1}^Nk=N\,\frac{N(N+1)}2-\frac{N(N+1)(2N+1)}6+\frac{N(N+1)}2=\frac{N(N+1)}2\left(N-\frac{2N+1}3+1\right)=\frac{N(N+1)(N+2)}6. $$

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    There is a typo. It should be N+2 in place of N+4.2012-11-29
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    @Gautam Shenoy - thanks , Martin Argerami - thanks!2012-11-29
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    @draks 10/6 = 1 floor function2012-11-29
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    how are you expanding $\sum_{k=1}^N kN$ to $N\,\frac{N(N+1)}2$ ?2015-06-13
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    It's the basic identity $\sum_{k=1}^Nk=\frac{N(N+1)}2$, multiplied by $N$.2015-06-13
4

Your sum can be written as

$$\sum_{k=1}^Nk(N-k+1) = N\sum_{k=1}^{N}k - \sum_{k=1}^N{k^2} + \sum_{k=1}^N{k}$$

$$ = \frac{N(N+1)^2}{2} - \frac{N(N+1)(2N+1)}{6}$$

3

A more complex but fun alternative solution using Finite Calculus:

$$ \begin{align}  \sum_{x=1}^N x \left[(N+1)-x\right] & = \sum_{1}^{N+1} \left[ Nx - x(x-1)\right] \delta x = \sum_{1}^{N+1} \left( Nx^{(1)} - x^{(2)} \right) \delta x \\ & = \left[ \frac{N}{2}x^{(2)} - \frac{1}{3} x^{(3)} \right]_{1}^{N+1} = \left[ \frac{N}{2}x(x-1) - \frac{1}{3} x(x-1)(x-2) \right]_{1}^{N+1}\\ & = \left[ \frac{N^2(N+1)}{2} - 0 \right] - \left[ \frac{1}{3}(N+1)N(N-1) - 0 \right] \\ & = \frac{N(N+1)(N+2)}{6}. \end{align} $$

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    @ffrenz : Thanks frenz!!2012-11-29