P183
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Maximum Product of Parts

ℹ️Published on Friday, 22nd February 2008, 05:00 pm; Solved by 5139;
Difficulty rating: 45%

Let $N$ be a positive integer and let $N$ be split into $k$ equal parts, $r = N/k$, so that $N = r + r + \cdots + r$.
Let $P$ be the product of these parts, $P = r \times r \times \cdots \times r = r^k$.

For example, if $11$ is split into five equal parts, $11 = 2.2 + 2.2 + 2.2 + 2.2 + 2.2$, then $P = 2.2^5 = 51.53632$.

Let $M(N) = P_{\mathrm{max} }$ for a given value of $N$.

It turns out that the maximum for $N = 11$ is found by splitting eleven into four equal parts which leads to $P_{\mathrm{max} } = (11/4)^4$; that is, $M(11) = 14641/256 = 57.19140625$, which is a terminating decimal.

However, for $N = 8$ the maximum is achieved by splitting it into three equal parts, so $M(8) = 512/27$, which is a non-terminating decimal.

Let $D(N) = N$ if $M(N)$ is a non-terminating decimal and $D(N) = -N$ if $M(N)$ is a terminating decimal.

For example, $\sum\limits_{N = 5}^{100} D(N)$ is $2438$.

Find $\sum\limits_{N = 5}^{10000} D(N)$.



Soluzione

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