# qr.rtr.inv

From lasso2 v1.2-0
by Martin Maechler

##### Reconstruct the Inverse of R'R from a QR Object

From a QR object, compute the inverse matrix which is implicitely (but not explicitly!) used to solve the underlying least squares problem.

##### Usage

`qr.rtr.inv(qr)`

##### Value

The $p \times p$ matrix $(R'R)^{-1}$
or equivalently, the inverse of $X'X$ (i.e. `t(X) %*% X`

in R).

##### See Also

##### Examples

```
(h3 <- 1/outer(0:5, 1:3, "+"))
rtr <- qr.rtr.inv(qr(h3))
all.equal(c(rtr %*% 1:3), solve(crossprod(h3), 1:3))
```

*Documentation reproduced from package lasso2, version 1.2-0, License: GPL version 2 or later*

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