function m = askew(v)
% 生成反对称矩阵
%
% Input: v - 3x1 vector
% Output: m - v的反对称阵
% | 0 -v(3) v(2) |
% m = | v(3) 0 -v(1) |
% |-v(2) v(1) 0 |
m = [ 0, -v(3), v(2);
v(3), 0, -v(1);
-v(2), v(1), 0 ];