Số nghiệm nguyên không âm của pt: $x+2y+5z=n$ là
$S_n=\sum_{y=0}^{\left\lfloor\frac n2\right\rfloor}\sum_{z=0}^{\left\lfloor\frac{n-2y}5\right\rfloor} 1 \quad \left(\textsf{hoặc }S_n=\sum_{z=0}^{\left\lfloor\frac n5\right\rfloor}\sum_{y=0}^{\left\lfloor\frac{n-5z}2\right\rfloor} 1\right)$
$S_n=\sum_{y=0}^{\left\lfloor\frac n2\right\rfloor} \left\lfloor\dfrac{n+5-2y}5\right\rfloor$
Đặt $n=10m+r; \;(0\le r \le 9)$
Đặt $y=5p+q;$ với $\,(0\le p\le m-1) \textsf{, và } (0\le q\le 4)$
Khi đó $\max y=5m-1$ còn thiếu đoạn cần lấy tổng $y=5m+t;\;\;\left(0\le t\le \left\lfloor\frac r2\right\rfloor\right) $
\begin{align*}S_n&=\sum_{p=0}^{m-1}\sum_{q=0}^4 \left\lfloor \dfrac{10m+r+5-10p-2q}{5}\right\rfloor +\sum_{t=0}^{\left\lfloor\frac r2\right\rfloor} \left\lfloor \dfrac{10m+r+5-10m-2t}{5}\right\rfloor \\ &= \sum_{p=0}^{m-1}\sum_{q=0}^4 \left(2m-2p+1+\left\lfloor \dfrac{r-2q}5 \right\rfloor \right)+ \sum_{t=0}^{\left\lfloor\frac r2\right\rfloor} \left\lfloor\dfrac{r+5-2t}5\right\rfloor \\ &= \sum_{p=0}^{m-1} (10m-10p+5+r-6)+S_r\qquad\textsf{(Hermite)}\\ &=\sum_{p=0}^{m-1}(n-1-10p)+S_r \\ &=(n-1)m-10\dfrac{(m-1)m}{2}+\{1,1,2,2,3,4,5,6,7,8\} \\ &=\dfrac{(n-1)(n-r)}{10} -\dfrac{(n-r)(n-10-r)}{20}+\{1,1,2,2,3,4,5,6,7,8\} \\ &= \dfrac{n^2+8n}{20}-\dfrac{r^2+8r}{20} +\{1,1,2,2,3,4,5,6,7,8\} \\ &=\dfrac{n^2+8n+\{20,11,20,7,12,15,16,15,12,7\}}{20} \\ &=\left\lfloor\dfrac{n^2+8n+20}{20}\right\rfloor \end{align*}
- Nobodyv3 yêu thích