R Under development (unstable) (2024-01-25 r85826 ucrt) -- "Unsuffered Consequences"
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> library(lfe)
Loading required package: Matrix
> options(digits=3)
> set.seed(42)
> B <- matrix(rnorm(40000),200)
> A <- band(crossprod(B), -30,30)
> tx <- rnorm(ncol(A))
> b <- A %*% tx
> fun <- function(x) A %*% x
> sol <- cgsolve(fun,b,eps=-0.001, symmtest=TRUE)
> sqrt(sum((sol-tx)^2))
[1] 0.000138
> 
> A[5,6] <- 0.24
> options(warn=2)
> try(cgsolve(fun,b,symmtest=TRUE))
Error in cgsolve(fun, b, symmtest = TRUE) : 
  (converted from warning) fun seems to be non-symmetric
> 
> proc.time()
   user  system elapsed 
   1.29    0.23    1.51