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Thanks. That does indeed cause the two outputs to match. I don't know if it's possible, but I'm still open to suggestions that don't involve manually adding space.

In the formula $$f \left( x \right)$$, there is some space inserted between the 'f' and '(', but in the formula $$f ( x )$$, there is no space. I know in this simple example I could just use the latter option, but this comes from a command which I use both for functions with regular sized arguments ...