Flexible polymer knots with strict topological constraint of no segment cro
ssing are studied by Monte Carlo simulations. The nonequilibrium relaxation
time of an equilibrated polymer knot cut at one point to relax to a linear
chain is measured. Prime knots up to 20 essential crossings from the group
s (3(1),5(1),7(1)....), (4(1), 6(1), 8(1),...) and (5(2), 7(2), 9(2)....),
(6(2), 8(2), 10(2)....) are studied. The nonequilibrium relaxation time for
knots within a group are found to increase stepwise linearly with the numb
er of essential crossings of the original knot. Our results suggest an equa
lly spaced topological interaction energy spectrum for knots in the same gr
oup and thus provide a quantitative description of topological interactions
.