In this paper the solutions $u_{ \nu}=u_{ \nu}(x,t)$ to fractional diffusion equations of order $0<\nu \leq 2$ are analyzed and interpreted as densities of the composition of various types of stochastic processes. For the fractional equations of order $\nu =\frac{ 1}{ 2^n}$, $n\geq 1,$ we show that the solutions $u_{ { 1/2^n}}$ correspond to the distribution of the $n$-times iterated Brownian motion. For these processes the distributions of the maximum and of the sojourn time are explicitly given. The case of fractional equations of order $\nu =\frac{ 2}{ 3^n}$, $n\geq 1,$ is also investigated and related to Brownian motion and processes with densities expressed in terms of Airy functions. In the general case we show that $u_{ \nu}$ coincides with the distribution of Brownian motion with random time or of different processes with a Brownian time. The interplay between the solutions $u_{ \nu}$ and stable distributions is also explored. Interesting cases involving the bilateral exponential distribution are obtained in the limit.
Categories: math.PR