# qr.rtr.inv

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##### 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.

Keywords
algebra, array
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).