Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optimal time-dependent lower bound on density for classical solutions of
  1-D compressible Euler equations   For the compressible Euler equations, even when the initial data are
uniformly away from vacuum, solution can approach vacuum in infinite time.
Achieving sharp lower bounds of density is crucial in the study of Euler
equations. In this paper, for the initial value problems of isentropic and full
Euler equations in one space dimension, assuming initial density has positive
lower bound, we prove that density functions in classical solutions have
positive lower bounds in the order of $\textstyle O(1+t)^{-1}$ and $\textstyle
O(1+t)^{-1-\delta}$ for any $\textstyle 0<\delta\ll 1$, respectively, where $t$
is time. The orders of these bounds are optimal or almost optimal,
respectively. Furthermore, for classical solutions in Eulerian coordinates
$(y,t)\in\mathbb{R}\times[0,T)$, we show velocity $u$ satisfies that
$u_{y}(y,t)$ is uniformly bounded from above by a constant independent of $T$,
although $u_{y}(y,t)$ tends to negative infinity when gradient blowup happens,
i.e. when shock forms, in finite time.

6
The categories of this paper are: astro-ph
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
1
The categories of this paper are: cs
18
The categories of this paper are: econ
21
The categories of this paper are: adap-org
16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
23
The categories of this paper are: solv-int