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4.14.6.1 Delta tensor
The delta tensor takes two indices, is symmetric and has the matrix
representation diag(1, 1, 1, ...). It is constructed by the function
delta_tensor():
{
symbol A("A"), B("B");
idx i(symbol("i"), 3), j(symbol("j"), 3),
k(symbol("k"), 3), l(symbol("l"), 3);
ex e = indexed(A, i, j) * indexed(B, k, l)
* delta_tensor(i, k) * delta_tensor(j, l);
cout << e.simplify_indexed() << endl;
// -> B.i.j*A.i.j
cout << delta_tensor(i, i) << endl;
// -> 3
}
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