This means that fiscal policy is sustainable if present discounted
value of expected future debt converges toward zero, i.e. if the present discounted
value of the share of primary deficit in output equals the negative current value of
the share of output. Accordingly, an indebted government sooner or later needs to
run a primary surplus big enough to meet the above equations (Heinemann, 1992;
Blanchard et al., 1990; Spaventa, 1987). Conditions (7) or (8) respectively, that
current value of future debt should equal zero, allows financing of the existing debt
by new borrowing ("rolling over" into the future), but in such case it requires that