**R – Xi'an's Og**, and kindly contributed to R-bloggers)

**O**nce the pseudo-story [noise] removed, a linear programming Le Monde mathematical puzzle:

For the integer linear programming problemmax 2x¹+2x²+x³+…+x¹⁰

under the constraintsx¹>x²+x³, x²>x³+x⁴, …, x⁹>x¹⁰+x¹, x¹⁰>x¹+x²

find a solution with the maximal number of positive entries.

**E**xpressed this way, it becomes quite straightforward to solve with the help of a linear programming R code like lpSolve. Indeed, a simple iteration of the constraints shows that positive entries are necessarily bracketed by negative entries, since, e.g.,

x²<-88x¹/55, x¹⁰<-33x¹/55

(which applies to all consecutive triplets since the constraints are invariant by transposition). Hence there are at most five positive entries but calling lpSolve with this option

> lp (direction="max", objective.in=c(2,2,rep(1,8)), const.mat=A, const.dir=rep(">=",10), const.rhs=rep(1,10)+A%*%c(rep(c(20,-1),5)), all.int=TRUE) Error: no feasible solution found

shows this is not possible. (The added vector is my way of getting around the constraint that *lpSolve only considers positive entries*. I therefore moved the negative entries by 20, meaning they are assumed to be larger than -20. Using the larger bound 50 does not change the outcome.) From there, there are 10 possible versions of vectors with four positive entries and a simple loop returns

> masume [1] -90 > topast [1] -11 1 -13 1 -15 1 -17 1 -19 -9

as the maximal criterion and argument of this maximum with four positive entries.

As an aside, the previous Le Monde puzzle [#1005] was also provided by a linear system: given 64 cubes, each of the 384 faces being painted in one of four colours, with exactly 40 of these cubes exhibiting the four colours, the question was about the number of cubes that could be bicolour so that a mono-colour super-cube could be reconstituted for all colours. Which amounted to solve the four equations

4a+2b=24,4c+2d=40, b+c=8,d+3a=24,

leading to 40 quadri-colour, 16 tri-colour, and 8 bi-colour cubes.

Filed under: Kids, R Tagged: competition, Le Monde, linear programming, linear system of equations, lpSolve, mathematical puzzle, optimisation, R

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**R – Xi'an's Og**.

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