People where buying billions of dollars of short term treasury bonds with a negative interest rate recently, because they are safer than mattress's. However, I was talking about real costs after inflation ex: Inflation = 2%, government bond's yield 2.5% capital gains = 25%, mattress yield = 0%. (1 + .025 * .75)/1.02 = 99.88% vs (1/1.02) = 98.04%
The only number the government does not control is the interest rate they need to pay.
Why will Japanese people buy government bonds that return a negative interest? Stuffing cash in a mattress seems like a better option at that point.