Sentence:  Further, the <WSD>Kronecker delta</WSD> and the permutation symbol are related by the identity, known as the e-\delta "identity" [see Problem 2.5(d)], / :::: e_{ijk} e_{imn} = \delta_{jm}\delta_{kn} - \delta_{jn}\delta_{km}.      (2.2.43) / The permutation symbol and the <WSD>Kronecker delta</WSD> prove to be very useful in proving vector identities. 
Meaning of the word marked with <WSD> tags:  A binary function, written as &delta; with two subscripts, which evaluates to 1 when its arguments are equal, and 0 otherwise.