Bibliographic Information: To appear in the Proceedings of the 44th Annual IEEE Symposium on Foundations of Computer Science.
 -by-
-by- symmetric positive semi-definite, diagonally dominant 
  matrix
 symmetric positive semi-definite, diagonally dominant 
  matrix  with
 with  non-zero entries and an
 non-zero entries and an  -vector
-vector  ,
  produces a vector
,
  produces a vector  within relative distance
 within relative distance  of the solution to
  of the solution to  in time
  in time 
 ,
  where
,
  where 
 is the log of the ratio of the largest to smallest non-zero
  eigenvalue of
 is the log of the ratio of the largest to smallest non-zero
  eigenvalue of  .
In particular,
.
In particular, 
 , where
, where
   is the logarithm of the ratio of the largest to smallest
  non-zero entry of
 is the logarithm of the ratio of the largest to smallest
  non-zero entry of  .
If the graph of
.
If the graph of  has genus
 has genus  or does not have a
  or does not have a 
 minor,
  then the exponent of
 minor,
  then the exponent of  can be improved to
  the minimum of
 can be improved to
  the minimum of
   and
 and 
 .
The key contribution of our work is an extension
  of Vaidya's techniques for constructing and
  analyzing combinatorial preconditioners.
.
The key contribution of our work is an extension
  of Vaidya's techniques for constructing and
  analyzing combinatorial preconditioners.