A note on the g -angle between subspaces of a normed space

We introduce a new 2-norm on a normed space using a semi-inner product g on the space. Using this 2-norm, we propose a formula for the g-angle between 2-dimensional subspaces in the space. Our formula serves as a revision of the one proposed by Nur et al. (Beitr. Algebra Geom. 59(1):133–143, 2018).