A relativistic method for the nonrelativistic gap problem

Saxon-Hutner theorem via matrix exponential

“…In the mean time Subramanian and Bhagwat [12] have transferred Luttinger's result into a relativistic situation and later on a general criterion which covers both relativistic and non-relativistic domains was derived in [13]. A number of such criteria and their interrelations have been discussed by Hermann [14]. The work of Erdös and Herndon [15] is a comprehensive review of methods, both analytical and numerical, for the non-relativistic variant of the Saxon-Hutner theorem as well as an extensive list of references.…”