i need to do logistic regression 20 times. The response variable is Y,
explanatory variables are x1-x20. For each pair of x(i), we will do the
logistic regression between （x(i),x(j)) and Y. The target is to find the
pair of x that obtain the largest AUC.

而且要把(x1,x1)类似的对山除掉。
Thanks!!!