A little research with Google would tell you that the trillion dollar platinum coin idea to pay of the national debt originated with BoGritz, a populislt candidate for president, in 1992. The idea was detailed in 2010 by lawyer Carlos Mucha who blogged as "beowulf" on Wall Street economist and hedge fund manager Warren Mosler's blog.   http://moslereconomics.com/

In 2013 Paul Krugman wrote that President Obama should be ready to mint a trillion dollar coin to transfer to the treasury in case Congressional Republicans decided to drive our nation into default, a Constitutionally prohibited action that congressional Republicans nevertheless threatened to do. Krugman called the coin "harmless".    https://krugman.blogs.nytimes.com/2013/01/07/be-ready-to-mint-that-coin/?_r=0

Why is it harmless? Any currency represents a debt owed to the bearer. The actual total debt, as I understand it, would be unchanged, 

You don't understand Krugman at all. If total ignorance is ok with you, so be it.