  We find d - 2 relative differential invariants for a d-web, d \geq 4, on a
two-dimensional manifold and prove that their vanishing is necessary and
sufficient for a d-web to be linearizable. If one writes the above invariants
in terms of web functions f (x,y) and g_4 (x,y),...,g_d (x,y), then necessary
and sufficient conditions for the linearizabilty of a d-web are two PDEs of the
fourth order with respect to f and g_4, and d - 4 PDEs of the second order with
respect to f and g_4,...,g_d. For d = 4, this result confirms Blaschke's
conjecture on the nature of conditions for the linearizabilty of a 4-web. We
also give Mathematica codes for testing 4- and d-webs (d > 4) for
linearizability and examples of their usage.
