I have some basic problems with tensors and Einstein summation notation. As I understand, if a suffix appears twice in a tensor then summation over repeated indices is assumed, i.e.
where is a second order tensor and is the repeated suffix. Is indirectly a zeroth order tensor, i.e. a scalar?
Einstein summation convention also states that a suffix cannot appear more than twice in an expression. I came across an expression for fourth order tensor , where it is mentioned that is a scalar. Why is a zeroth order tensor, i.e. a scalar? I would not expect that since two different suffices are used.