  Bitcoin and other similar digital currencies on blockchains are not ideal
means for payment, because their prices tend to go up in the long term (thus
people are incentivized to hoard those currencies), and to fluctuate widely in
the short term (thus people would want to avoid risks of losing values). The
reason why those blockchain currencies based on proof of work are unstable may
be found in their designs that the supplies of currencies do not respond to
their positive and negative demand shocks, as the authors have formulated in
our past work. Continuing from our past work, this paper proposes minimal
changes to the design of blockchain currencies so that their market prices are
automatically stabilized, absorbing both positive and negative demand shocks of
the currencies by autonomously controlling their supplies. Those changes are:
1) limiting re-adjustment of proof-of-work targets, 2) making mining rewards
variable according to the observed over-threshold changes of block intervals,
and 3) enforcing negative interests to remove old coins in circulation. We have
made basic design checks and evaluations of these measures through simple
simulations. In addition to stabilization of prices, the proposed measures may
have effects of making those currencies preferred means for payment by
disincentivizing hoarding, and improving sustainability of the currency systems
by making rewards to miners perpetual.
