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**Basic numeracy - exact meaning of "accurate to one part in a thousand"**

## Homework Statement

I have a standard Coulomb model for the ground-state energy of a hydrogen atom. I have used peturbation theory to calculate a (fictitious) first order correction to the model, which contains a constant a.

I now have to find the largest value of a which will be consistent with the fact that the Coulomb model is accurate to one part in a thousand.

It's a basic numeracy problem, despite being a complicated topic. Does accurate to one part in a thousand mean that the correction shouldn't contribute more than +/- 0.001 J to the energy calculated using the Coulomb model, or does it mean it shouldn't add more than +/- 0.00049999 J?

A bit basic, I know, but I don't want to fail at this final hurdle.

Thanks in advance.

## Homework Equations

##E = E_c + \delta E##