  We study the existence and nonexistence of a Cauchy problem of the semilinear
heat equation $\partial_tu=\Delta u+|u|^{p-1}u$ in $\mathbb{R}^N\times(0,T)$,
$u(x,0)=\phi(x)$ in $\mathbb{R}^N$, in $L^1(\mathbb{R}^N)$. Here, $N \ge 1$,
$p=1+2/N$ and $\phi\in L^1( \mathbb{R}^N)$ is a possibly sign-changing initial
function. Since $N(p-1)/2=1$, the $L^1$ space is scale critical and this
problem is known as a doubly critical case. It is known that a solution does
not necessarily exist for every $\phi\in L^1(\mathbb{R}^N)$. Let $X_q:=\{
\phi\in L^1_{\rm{loc}}(\mathbb{R}^N)\ |\ \int_{\mathbb{R}^N}|\phi| \left[\log
(e+|\phi|)\right]^qdx<\infty \} (\subset L^1(\mathbb{R}^N))$. In this paper we
construct a local-in-time mild solution in $L^1(\mathbb{R}^N)$ for $\phi\in
X_q$ if $q\ge N/2$. We show that, for each $0\le q<N/2$, there is a nonnegative
initial function $\phi_0\in X_q$ such that the problem has no nonnegative
solution, using a necessary condition given by Baras-Pierre [Ann. Inst. H.
Poincar\'{e} Anal. Non Lin\'{e}aire 2 (1985), 185--212]. Since $X_q\subset
X_{N/2}$ ($q\ge N/2$), $X_{N/2}$ becomes a sharp integrability condition. We
also prove a uniqueness in a certain set of functions which guarantees the
uniqueness of the solution constructed by our method.
