i was trying to tell it like the following -

```
c1:= \partial_{\alpha}{\partial_{\beta}{h_{\mu \nu}} =
\partial_{\beta}{\partial_{\alpha}{h_{\mu \nu}};
```

its showing error " free index is not matching" something like that

+1 vote

That one had me puzzled for a minute... The solution is to match your brackets: both terms should have an additional `}`

to close the argument of the outmost derivative. So

```
\partial{#}::PartialDerivative;
c1:= \partial_{\alpha}{\partial_{\beta}{h_{\mu \nu}}} =
\partial_{\beta}{\partial_{\alpha}{h_{\mu \nu}}};
```

And to answer the question in the subject: you can then make these terms equal with a call to `canonicalise(c1)`

.

...