X=[X;fliplr(x(i:2*N+i))]

X=[X;fliplr(x(i:2*N+i))]
X=[X;fliplr(x(i:2*N+i))]

X=[X;fliplr(x(i:2*N+i))]
方括号中的;表示拼接矩阵
fliplr函数实现矩阵的左右翻转；也就是把x矩阵的第i个到2*N+i个元素沿垂直轴左右翻转