How find this inequality minimum $\sum_{i=1}^{n}a^2_{i}-2\sum_{i=1}^{n-1}a_{i}a_{i+1}$

Assume that $n$ is give positive integer numbers,and let $a_{1},a_{2},\cdots,a_{n}\ge 0$,such


Find this minimum value


I think maybe use this well kown

$$a^2_{1}+a^2_{2}+\cdots+a^2_{n}\ge a_{1}a_{2}+a_{2}a_{3}+\cdots+a_{n-1}a_{n}+a_{n}a_{1}?$$

But this problem is only
so I can’t it.Thank you for help.

By the way:
I don’t know this problem have Someone research?if No,I think this is interesting problem.

Solutions Collecting From Web of "How find this inequality minimum $\sum_{i=1}^{n}a^2_{i}-2\sum_{i=1}^{n-1}a_{i}a_{i+1}$"

I show below that the minimum is $-\frac{1}{6}$ for $n \geq 4$, and $-\frac{1}{7}$ when $n = 3$.

If we put

Q_n(a_1,a_2,\ldots ,a_n)=


R_n(a_1,a_2,\ldots ,a_n)=Q_n(a_1,a_2,\ldots,a_n)+\frac{(a_1+a_2+a_3+\ldots +a_n)^2}{6}

then for $n\geq 4$, we have the identity


Since $Q_n(0,\ldots, 0,\frac{1}{6},\frac{1}{3},\frac{1}{3},\frac{1}{6})=-\frac{1}{6}$,
we see that the minimum is $-\frac{1}{6}$ for $n\geq 4$.

For $n=3$, the minimum is $-\frac{1}{7}$ attained at
$(\frac{2}{7},\frac{3}{7},\frac{2}{7})$ (thanks Macavity) because of


You almost did the problem yourself, just left out a few bits:

Assume, without loss of generality,

$$a_1 \geq a_2 \geq \dots \geq a_n$$

By the rearrangement inequality,

$$\implies a_1^2 + a_2^2 \dots a_n^2 \geq a_1a_2 + a_2a_3 \dots a_{n-1}a_n + a_na_1$$

Clearly, the required will be minimum if equality holds true above.

This will happen when

$$a_1 = a_2 = a_3 = \dots = a_n = a$$

This means, the maximum value of the inequality is:

$$\sum_{i = 1}^n a^2 – 2a^2 = -na^2$$

Since, from the first equation,

$$na = 1$$

$$a = \frac{1}{n}$$

Therefore, the minimum is,

$$-n\cdot\left(\frac{1}{n^2}\right) = -\frac{1}{n}$$