Which inequality?

Algebra Level 4

If $x_1,~x_2,~x_3,\dots,x_n$ are $n$ non-zero real numbers such $(x_1^2+x_2^2+x_3^2+\dots+x_{n-1}^2)(x_2^2+x_3^2+x_4^2+\dots+x_n^2)\leq(x_1x_2+x_2x_3+x_3x_4+\dots+x_{n-1}x_n)^2$ then $x_1,~x_2,~x_3,\dots,x_n$ are in

Note:
- AP denotes an arithmetic progression.
- AGP denotes an arithmetic geometric progression.
- GP denotes an geometric progression.
- HP denotes an harmonic progression.