x1=rnorm(20)

x2=rnorm(20)

x3=2*x1

x4=rnorm(20)

x5=3*x4

y=0.18*x1-2.23*x2+1.22*x3+2.62*x4-1.79*x5

data.frame(x1,x2,x3,x4,x5)

a=data.frame(x1,x2,x3,x4,x5)

cor(a)

library(car)              

attach(a)

fm=lm(y~.,data=a)      

到vif这一步会显示：      

vif(fm)

Error in vif.default(fm) : there are aliased coefficients in the model

r软件刚入门，求问这是什么原因？怎么解决？