Semismall perturbations, semi-intrinsic ultracontractivity, and integral representations of nonnegative solutions for parabolic equations

We consider nonnegative solutions of a parabolic equation in a cylinder $D \timesI$,where$D$isanoncompactdomainofaRiemannianmanifoldand$I =(0,T)$with$0 < T \le \infty$or$I=(-\infty,0)$. Under the assumption [SSP] (i.e., the constant function 1 is a semismall perturbation of the associated elliptic operator on $D$), we establish an integral representation theorem of nonnegative solutions: In the case $I=(0,T)$, any nonnegative solution is represented uniquely by an integral on $(D \times {0 }) \cup (\partial_M D \times [0,T))$,where$\partial_M D$istheMartinboundaryof$D$ for the elliptic operator; and in the case $I=(-\mathrm{\infty },0)$, any nonnegative solution is represented uniquely by the sum of an integral on $\partial_M D \times (-\infty,0)$ and a constant multiple of a particular solution. We also show that [SSP] implies the condition [SIU] (i.e., the associated heat kernel is semi-intrinsically ultracontractive).