with A1 is nonsingular and take A1 as preconditioner. However, for rank deficient case, we cannot obtain splitting (2.3) with nonsingular A1. The natural way is to consider full rank splitting (2.3) with A1 2 Rrn where
r ¼ rankðAÞ. Now we have to find the right preconditioner to construct block preconditioned SOR methods
for solving (2.1). From fundamental theorem of linear algebra, we know that any x 2 Rn, x ¼ ATy þ z where
z 2 NðAÞ. If we find RðATÞ ¼ RðAT 1 Þ, then we can construct preconditioner A1AT 1 which is nonsingular. For this
purpose, Lemma 2.1 in [33], that is RðAT 1 Þ ¼ RðATÞ and NðAÞ ¼ NðA1Þ, offered the required result. Thus, based
on the result, the block preconditioned SOR methods and block preconditioned conjugate gradient method
are established for solving problem (2.2) for rank deficient A in [33,34]. The convergence theory of such methods was also studied.