  There is a notion of non-commutative Lie algebra called "Leibniz algebra",
which is characterized by the condition: left bracketing is a derivation. The
purpose of this article is to introduce and study a new notion of algebra,
called "associative dialgebra", which is, to the Leibniz algebras, what
associative algebras are to Lie algebras. An associative dialgebra is a vector
space equipped with two associative operations satisfying three more
conditions. For instance, any differential associative algebra gives rise to a
dialgebra.
  In this article we construct and study a (co)homology theory for associative
dialgebras. The surprizing fact, in the construction of the chain complex, is
the appearance of the combinatorics of planar binary trees (grafting and
nesting). The principal result about this homology theory is its vanishing on
free associative dialgebras.
  The Koszul dual (in the sense of Ginzburg and Kapranov) of the operad of
associative dialgebras is the operad of "dendriform algebras". The dendriform
algebras are characterized by two operations satisfying three linear
conditions. The sum of these two operations defines a new operation which is
associative. The free dendriform algebra can be described in terms of planar
binary trees. As a consequence we give an explicitly description of strong
homotopy associative dialgebras.
  This paper is part of a long-standing project whose ultimate aim is to study
periodicity phenomenons in algebraic K-theory, as explained in ``Overview on
Leibniz algebras, dialgebras and their homology". Fields Inst. Commun. 17
(1997), 91--102.
