说明：  用IDL语言编写的计算实对称矩阵的的最大特征值与特征向量，可在主成分分析和偏最小二乘分析中应用。
(the procedure uses IDL language to calculate the biggest eigenvalues ​ ​ and corresponding eigenvectorsof a real symmetric matrix A , and is very useful for principal component analysis and partial least squares analysis applications.)