Le Monde puzzle

The puzzle in Le Monde is quite straightforward (!) this weekend: it can be rewritten as to figure out the values of the sums

10! \displaystyle{\left.\left\{1 - \sum_{j_1=1}^{10} \frac{1}{j_1}\left\{1-\sum_{\stackrel{j_2=1}{j_2\ne j_1}}^{10}\frac{1}{j_2}\right\{1-\ldots\right.\right.}

\displaystyle{\qquad\ldots\left.\left.\left.\left\{1-\sum_{\stackrel{j_9=1}{j_9\ne j_1,\ldots,j_8}}^{10} \frac{1}{j_9}\right\}\ldots\right\}\right\}\right\}}


\displaystyle{\left.(49!)^2 \left\{1 - \sum_{j_1=1}^{49} \frac{1}{j_1^2}\left\{1-\sum_{\stackrel{j_2=1}{j_2\ne j_1}}^{49}\frac{1}{j_2^2}\right\{1-\ldots\qquad\right.\right.}

\displaystyle{\qquad\ldots\left.\left.\left.\left\{1-\sum_{\stackrel{j_{48}=1}{j_{48}\ne j_1,\ldots,j_{47}}}^{49} \frac{1}{j_{48}^2}\right\}\ldots\right\}\right\}\right\}}

which are easily displayed and as easily solved.

The first sum can indeed be written as

\displaystyle{\sum_{i=1}^k (-1)^{k-i} \sum_{\stackrel{j_1\ne\cdots\ne j_i}{\in\{1,\ldots,k\}}} \prod_{u=1}^i j_u}

for k=10. This is simply

\displaystyle{\sum_{i=0}^k (-1)^{k-i} \sum_{\stackrel{j_1\ne\cdots\ne j_i}{\in\{1,\ldots,k\}}} \prod_{u=1}^i j_u}- (-1)^k = \prod_{i=1}^k (i-1) - (-1)^k

and the solution is thus (-1)^{k+1}, equal to -1 for k=10. Once we realise the fact this is a product with one missing term, the second sum is similar: it can be written as

\displaystyle{\sum_{i=0}^k (-1)^{k-i} \sum_{\stackrel{j_1\ne\cdots\ne j_i}{\in\{1,\ldots,k\}}} \prod_{u=1}^i j_u^2}- (-1)^k

\qquad = \prod_{i=1}^k (i^2-1) - (-1)^k = - (-1)^k

This is equal to 1 for k=49. A bit disappointing because it amounts to reformulate the question with the proper algebraic formula…

4 Responses to “Le Monde puzzle”

  1. […] Monde puzzle [52] The last puzzle of the year in Le Monde reads as follows (as far as I understand its wording!): Iter(n,x,y) is the […]

  2. […] Le Monde puzzle When I first read Le Monde puzzle this weekend, I though it was even less exciting than the previous one: […]

  3. […] Robin Ryder started his new blog with his different solutions to Le Monde puzzle, solutions that are much more elegant than my pedestrian rendering. I particularly like the one […]

  4. […] Monde puzzle on sums of products By robinryder Christian Robert is disappointed by last week-end’s Le Monde’s mathematical problem : Take the integers 1 to 10. Group […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: