1. The problem statement, all variables and given/known data I am self studying relativity. In Wikipedia under the four-gradient section, the contravariant four-vector looks wrong from an Einstein summation notation point of view. https://en.wikipedia.org/wiki/Four-vector 2. Relevant equations It states: E0∂0-E1∂1-E2∂2-E3∂3 = Eα∂α 3. The attempt at a solution As it is it looks wrong to me. Is it wrong or is it NOT ESN or something else?