We present a solution of the algebraic version of Birkhoff Conjecture on
integrable billiards. Namely we show that every polynomially integrable real
bounded convex planar billiard with smooth boundary is an ellipse. We extend
this result to billiards with piecewise-smooth and not necessarily convex
boundary on arbitrary two-dimensional surface of constant curvature: plane,
sphere, Lobachevsky (hyperbolic) plane; each of them being modeled as a plane
or a (pseudo-) sphere in R3 equipped with appropriate quadratic
form. Namely, we show that a billiard is polynomially integrable, if and only
if its boundary is a union of confocal conical arcs and appropriate geodesic
segments. We also present a complexification of these results. These are joint
results of Mikhail Bialy, Andrey Mironov and the author. The proof is split
into two parts. The first part is given by Bialy and Mironov in their two joint
papers. They considered the tautological projection of the boundary to
RP2 and studied its orthogonal-polar dual curve, which is piecewise
algebraic, by S.V.Bolotin's theorem. By their arguments and another Bolotin's
theorem, it suffices to show that each non-linear complex irreducible component
of the dual curve is a conic. They have proved that all its singularities and
inflection points (if any) lie in the projectivized zero locus of the
corresponding quadratic form on C3. The present paper provides the
second part of the proof: we show that each above irreducible component is a
conic and finish the solution of the Algebraic Birkhoff Conjecture in constant
curvature.Comment: To appear in the Journal of the European Mathematical Society (JEMS),
69 pages, 2 figures. A shorter proof of Theorem 4.24. Minor precisions and
misprint correction