A note on extreme points in the closed unit ball of upper triangular 2 × 2 matrices over a C*-algebra

Given a unital C*-algebra A, letMm?n(A) be the set of all m?n matrices algebra over A and (Mn(A))1 closed unit ball Mn?n(A). Let x =(a b 0 c) ? (Mm+n(A))1 determined by Mm?m(A), Mm?n(A) c Some characterizations are given such that above upper triangular matrix is an extreme point Xm,n(A) respectively, where subset (Mm+n(A)) 1 consisting matrices.