[Rd] ?eigen documentation suggestion

Dan Davison davison at stats.ox.ac.uk
Thu Nov 29 12:34:18 CET 2007

from ?eigen

symmetric: if 'TRUE', the matrix is assumed to be symmetric (or
          Hermitian if complex) and only its lower triangle is used. If
          'symmetric' is not specified, the matrix is inspected for

I think that could mislead a naive reader as it suggests that, with symmetric=TRUE,
the result of eigen() (vectors and values) depends only on the lower-triangular values, 
and thus not on the values on the diagonal (that's not true for the eigenvalues).

I suggest that changing it to 

"... and only its lower triangle is used when computing the eigenvectors."

would be more accurate, perhaps even with a reminder about the trace - sum of eigenvalues


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