[R] ginv vs. solve

[Uwe Ligges]

Angel wrote:

> Why do
> x<-b%*%ginv(A)

Why should it be the same???
Here, you are calculating  (A+ is G-Inverse)

   x = b * A+

> and
> x<-solve(A,b)

Here, you are calculating (A- is Inverse of A)

   A * x = b  <=> A- * A * x = A- * b <=>
   x = A- * b

So you have to compare, e.g.:

  ginv(A) %*% b
  solve(A, b)

Uwe Ligges


> give different results?. It seems that I am missing some basic feature of
> matrix indexing.
> e.g.:
> 
> A<-matrix(c(0,-4,4,0),nrow=2,ncol=2)
> b<-c(-16,0)
> x<-b%*%ginv(A);x
> x<-solve(A,b);x
> 
> Thanks in advance,
> Angel
> 
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