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Given that total outstanding US Treasury and agency debt in public hands was about $6 trillion at the end of 2006

С учетом того, что совокупная сумма долговых обязательств Минфина и государственных организаций США, находящихся в свободном обращении на рынках, на конец 2006 года составила ...

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Note added at 28 mins (2014-10-03 12:54:59 GMT)
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total outstanding US Treasury and agency debt = совокупная сумма долговых обязательств Минфина и государственных организаций США (это долговые ценные бумаги -- прим. перев.);

outstanding = обращающихся; находящихс в обращении; выпущенных в обращение;

in public hands = находящихся в свободном обращении; свободно обращающихся на биржах/рынках.

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Note added at 1 hr (2014-10-03 14:24:44 GMT)
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Аргументация:

Gross Federal Debt vs. Net Debt

The US federal government differentiates between Gross Debt issued by the US Treasury and Net Debt held by the public. The numbers on Gross Debt are published by the US Treasury here.

Numbers on various categories of federal debt, including Gross Debt, debt held by federal government accounts, debt held by the public, and debt held by the Federal Reserve System, are published every year by the Office of Management and Budget in the Federal Budget in the Historical Tables as Table 7.1 — Federal Debt at the End of the Year. The table starts in 1940. You can find the latest Table 7.1 in here.

The chart above shows three categories of federal debt.
1. Monetized debt (blue), i.e., federal debt bought by the Federal Reserve System
2. Debt held by the federal government (red) e.g., as IOUs for Social Security
3. Other debt (green), i.e., debt in public hands, including foreign governments.

http://www.usgovernmentspending.com/federal_debt