Debt-to-GDP ratios are standard economic metrics where nominal GDP serves as the denominator to measure a country's total public debt relative to its overall economic output; no formal citation is needed for this definitional fact.
The claim states a standard economic definition regarding how debt-to-GDP ratios are calculated (using nominal GDP as the denominator). This is a matter of basic economic terminology and definition, making it common knowledge. No retrieved papers specifically test or dispute this standard accounting definition, as papers mentioning debt-to-GDP ratios simply utilize the standard formula in their background or methodology.