Algebraic 2-valued group structures on $\mathbb P^1$, Kontsevich-type polynomials, and multiplication formulas. I

The theory of a two-valued algebraic group structure on a complex plane and complex projective line is developed. In this theory, depending on the choice of the neutral element, the local multiplication law is given by the Buchstaber polynomial or the generalized Kontsevich polynomial. One of the most exciting results of our studies is a simple construction of a two-valued algebraic group on C different from known coset-construction.