A treatment of the Cauchy–Schwarz inequality in C⁎-modules

We study the Cauchy–Schwarz and some related inequalities in a semi-inner product module over a C⁎-algebra A. The key idea is to consider a semi-inner product A-module as a semi-inner product A-module with respect to another semi-inner product. In this way, we improve some inequalities such as the Ostrowski inequality and an inequality related to the Gram matrix. The induced semi-inner products are also related to the notion of covariance and variance. Furthermore, we obtain a sequence of nested inequalities that emerges from the Cauchy–Schwarz inequality. As a consequence, we derive some interesting operator-theoretical corollaries. In particular, we show that the sequence arising from our construction, when applied to a positive invertible element of a C⁎-algebra, converges to its inverse.