  In the 1970s, William Tutte developed a clever algebraic approach, based on
certain "invariants", to solve a functional equation that arises in the
enumeration of properly colored triangulations. The enumeration of plane
lattice walks confined to the first quadrant is governed by similar equations,
and has led in the past 20 years to a rich collection of attractive results
dealing with the nature (algebraic, D-finite or not) of the associated
generating function, depending on the set of allowed steps, taken in $\{-1,
0,1\}^2$.
  We first adapt Tutte's approach to prove (or reprove) the algebraicity of all
quadrant models known or conjectured to be algebraic. This includes Gessel's
famous model, and the first proof ever found for one model with weighted steps.
To be applicable, the method requires the existence of two rational functions
called invariant and decoupling function respectively. When they exist,
algebraicity follows almost automatically.
  Then, we move to a complex analytic viewpoint that has already proved very
powerful, leading in particular to integral expressions for the generating
function in the non-D-finite cases, as well as to proofs of non-D-finiteness.
We develop in this context a weaker notion of invariant. Now all quadrant
models have invariants, and for those that have in addition a decoupling
function, we obtain integral-free expressions for the generating function, and
a proof that this series is D-algebraic (that is, satisfies polynomial
differential equations).
