  For 1-D semilinear Tricomi equation $\partial_t^2 u-t\partial_x^2u=|u|^p$
with initial data $(u(0,x), \partial_t u(0,x))$ $=(u_0(x), u_1(x))$, where
$t\ge 0$, $x\in\mathbb{R}$, $p>1$, and $u_i\in C_0^\infty(\mathbb{R})$
($i=0,1$), we shall prove that there exists a critical exponent $p_{\rm
crit}=5$ such that the small data weak solution $u$ exists globally when
$p>p_{\rm crit}$; on the other hand, the weak solution $u$, in general, blows
up in finite time when $1<p<p_{\rm crit}$. We specially point out that for 1-D
semilinear wave equation $\partial_t^2 v-\partial_x^2v=|v|^p$, the weak
solution $v$ will generally blow up in finite time for any $p>1$. By this paper
and \cite{HWYin1}-\cite{HWYin3}, we have given a systematic study on the blowup
or global existence of small data solution $u$ to the equation $\partial_t^2
u-t\Delta u=|u|^p$ for all space dimensions. One of the main ingredients in the
paper is to establish a crucial weighted Strichartz-type inequality for 1-D
linear degenerate equation $\partial_t^2 w-t\partial_x^2 w=F(t,x)$ with
$(w(0,x), \partial_t w(0,x))=(0,0)$, i.e., an inequality with the weight
$(\frac{4}{9}t^3-|x|^2)^{\alpha}$ between the solution $w$ and the function $F$
is derived for some real numbers $\alpha$.
