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On touch surfaces because of the inaccuracy of touch it is hard to select nested strings. What I am thinking about is an object like
where if the picture is small enough when the user touches "b" it is unclear whether she ment "b" or "a+b" or "(a+b)" or "(a+b)^2".
What is the best way to provide the user a precise selection?
About the different solutions you propose:
Enlarging can help but you still need an extra step to ensure the limits of the selection. Lets say after zooming the user clicks on the + sign, does that mean he wants to select +, a+b, (a+b)?
Blow it to pieces could work if you have a number of predefined selections to give the user to choose from. Otherwise it can become complex: a, +, b, (, ), (a, a+, +b, etc.
Provide disjoint selection areas is the same as the former option, with a set of predefined selections. Visually more interesting. But the former is more clear in the sense that the user just needs to select from a list in a more ordered way without relying on color (which might cause confusion with contrast, color blind, etc.). The user already knows what he wants to select so there is no need to clarify this visually (although maybe in the case where a selection is repeated,for example (b+b)).
So whether you are simply selecting text or a different content, I think the key point is the selection limits rather than the exact point of the interaction.
Text selection on touch devices provides the user two handlers for him to determine the limits of the selection. The way it is approched is highlighting a certain part of text and giving the user the option to drag the start and end handlers. It gives the user more control over where his selection starts and where it finishes.
I agree with Alvaro on almost everything mentioned in his answer.
Another thing to take into consideration, what if the actual equation was much larger/ complex, and there were way more elements that needed to be selected.
In that case, both of the approaches mentioned might become cumbersome and slightly messy.
Why not, give one touch/ edit button/ option. Clicking on this opens an editor with zoomed in text, and the user can start editing directly from there. Something similar to using android keyboard. When you enter an incorrectly non-dictionary word, it allows you to add it to the dictionary; but opens an editor before you save the new word. In your case, the editor should ensure that the text size is larger than normal, for the user to easily select the element he desires.
[Please correct me if am wrong but my assumption here is that the only use case you are referring to is select an element and then edit the same]
