check all x = p/q, rational numbers ( -100 <= p <= 100,  1 <= q <= 100),
find P'(x) = 0  roots , (v[0], v[1], .. v[k] ->  P'(v[i]) = 0).
try find roots with binary search in [ v[i] .. v[i+1] ] !!!!!

http://en.wikipedia.org/wiki/Casus_irreducibilis
http://en.wikipedia.org/wiki/Rational_root_test
http://en.wikipedia.org/wiki/Cubic_function