  The relationship between the complexity classes $P$ and $NP$ is an unsolved
question in the field of theoretical computer science. In the first part of
this paper, a lattice framework is proposed to handle the 3-CNF-SAT problems,
known to be in $NP$. In the second section, we define a multi-linear descriptor
function ${\cal H}_\varphi$ for any 3-CNF-SAT problem $\varphi$ of size $n$, in
the sense that ${\cal H}_\varphi : \{0,1\}^n \rightarrow \{0,1\}^n$ is such
that $Im \; {\cal H}_\varphi$ is the set of all the solutions of $\varphi$. A
new merge operation ${\cal H}_\varphi \bigwedge {\cal H}_{\psi}$ is defined,
where $\psi$ is a single 3-CNF clause. Given ${\cal H}_\varphi$ [but this can
be of exponential complexity], the complexity needed for the computation of $Im
\; {\cal H}_\varphi$, the set of all solutions, is shown to be polynomial for
hard 3-CNF-SAT problems, i.e. the one with few ($\leq 2^k$) or no solutions.
The third part uses the relation between ${\cal H}_\varphi$ and the indicator
function $\mathbb{1}_{{\cal S}_\varphi}$ for the set of solutions, to develop a
greedy polynomial algorithm to solve hard 3-CNF-SAT problems.
