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