Reference: Moore, Thomas A., A General Relativity Workbook, University Science Books (2013) – Chapter 6; 5.
A specific case of the trace of a tensor is the trace of the metric tensor, which is given by . Since is the inverse of the metric tensor , is the identity matrix, which means it is diagonal with every diagonal element equal to 1. The trace of the identity matrix is simply , the dimension of the matrix. Thus in 2-d it would be and so on.