btw, the citizens also have a debt of 15.8 trillion. of wich a large amount is from foreign holders.
That's the combined public and intra-national debt. It's all very confusing, but it works something like this:
National governments take out loans in whatever form (most often Treasury securities) to provide funding for the welfare of their citizens. The nation in question then taxes its citizens to gain the funds necessary to pay back the loans. This is their sole income. Governments are not businesses, they do not sell services. So a national debt functions as a taxable citizens debt. It's not the debt of any individual citizen, and should a government default, it cannot be collected from any individual or group of citizens. Instead, the nation's currency drops in value. The public debt is the debt owed by the national government to parties not part of the national government (both native and foreign), while intra-national debt is money owed by one sector of the government to another sector of the government.
However, any individual citizen or corporation indebted to a specific creditor is held accountable by that specific creditor for their individual debts. As such, this type of debt cannot be divided per capita.
Furthermore, when a corporate creditor which also owes debts to another creditor defaults, that corporations creditors cannot be held accountable for their creditor's default and their debt does not necessarily transfer.
So while that $15.8 trillion (actually, more like $16.2 trillion) debt is owed by the citizens of the United States in theory, it's not actually owed by any individual United States citizen and cannot be collected from any individual or group of citizens.
I hope that made sense.
Note: I've always been more interested in the economics of scarcity and microeconomics than financial economics, so while I'm trying to be as accurate as possible, if I'm wrong feel free to correct me.