问题 Bellow is a stock daily returns matrix example ( ret_matriz ) IBOV PETR4 VALE5 ITUB4 BBDC4 PETR3 [1,] -0.040630825 -0.027795652 -0.052643733 -0.053488685 -0.048455772 -0.061668282 [2,] -0.030463489 -0.031010237 -0.047439725 -0.040229625 -0.030552275 -0.010409016 [3,] -0.022668170 -0.027012078 -0.022668170 -0.050372843 -0.080732363 0.005218051 [4,] -0.057468428 -0.074922051 -0.068414670 -0.044130126 -0.069032911 -0.057468428 [5,] 0.011897277 -0.004705891 0.035489885 -0.005934736 -0.006024115