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Problem: Let $n$ and $y$ be two positive integers such that $y\leq n!$.Then prove that there exist unique integers $x_1,x_2,...,x_{n-1}$ such that $0\leq x_i\leq n-i$ for $i=1,2,3,...,n-1$ and $y=1+ \sum_{i=1}^{n-1} x_i(n-i)!$.

My Proof: Let $(x_1,x_2,...,x_{n-1})$ be such a tuple ,then the number of all possible tuples satisfying $0\leq x_j\leq n-j$ for $j=1,2,...n-1$ is $n!$ and the number of all possible values of $y$ is also $n!$. The tuple corresponding to $y=1$ is $(0,0,...,0)$ and tuple corresponding to $y=n!$ is $(n-1,n-2,...,[n-(n-1)])$. All other tuples will correspond to some integer $1

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    The proof seems correct to me. It is surely not a famous problem. – 2017-01-03

1 Answers 1

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Another proof will be picking the minimum $k$.

Assuming $d_k > r_k$. Since $k$ is minimum, $n-k$ is maximum.

Also note that $\sum_{i=1}^{n-1} i*i! = n!-1$.

Therefore $(d_k-r_k) (n-k)! =\sum_{i=k+1}^{n-1} (r_i-d_i) (n-i)! \geq (n-k)!$.

However, $\sum_{i=k+1}^{n-1} (r_i-d_i) (n-i)! \leq \sum_{i=1}^{n-k-1} (i) (i)! = (n-k)!-1$. Contradiction.