  Let $p$ and $q$, where $pq-qp=1$, be the standard generators of the first
Weyl algebra $A_1$ over a field of characteristic zero. Then the spectrum of
the inner derivation $ad(pq)$ on $A_1$ are exactly the set of integers. The
algebra $A_1$ is a $\mathbb{Z}$-graded algebra with each $i$-component being
the $i$-eigenspace of $ad(pq)$, where $i\in \mathbb{Z}$. The Dixmier Conjecture
says that if some elements $z$ and $w$ of $A_1$ satisfy $zw-wz=1$, then they
generate $A_1$. We show that if either $z$ or $w$ possesses no component
belonging to the negative spectrum of $ad(pq)$, then the Dixmier Conjecture
holds. We give some generalization of this result, and some other useful
criterions for $z$ and $w$ to generate $A_1$. An important tool in our proof is
the Newton polygon for elements in $A_1$.
