## Find max abs off diag entry

n = nrow(A)

mx = abs(A[2,1])

mi = 2; mj = 1

for(i in 2:n) {

for(j in 1:(i-1)) {

if(mx < abs(A[i,j])) {

mi = i; mj = j; mx = abs(A[i,j]);

}

}

}

cat(mx,"\n")

## Compute theta

theta = atan2(2*A[mi,mj] , A[mj,mj]-A[mi,mi])/2

## Create rotation matrix

sn = sin(theta); cs = cos(theta)

G = diag(n); G[c(mi,mj),c(mi,mj)] = c(cs,sn,-sn,cs)

## Do the transformation

G %*% A %*% t(G)