  We find relative differential invariants of orders eight and nine for a
planar nonparallelizable 3-web such that their vanishing is necessary and
sufficient for a 3-web to be linearizable. This solves the Blaschke conjecture
for 3-webs. As a side result, we show that the number of linearizations in the
Gronwall conjecture does not exceed fifteen and give criteria for rigidity of
3-webs.
