“…These applications have often employed two-parameter Wiener processes, or Brownian motion, {W (x, t), x ∈ X, t ∈ T }, with mean zero and covariance Cov[W (x, s), W (y, t)] = min(x, y) min(s, t) where X and T are sub-intervals of R or R + (or their formal derivatives, space-time white noise {w(x, t)}). Such two-parameter processes first appeared over 50 years ago in works by Kitagawa (1951Kitagawa ( ),Čencov, (1956 and Yeh (1960). Zimmerman (1972) constructed a stochastic integral with respect to W and obtained a solution to what that author called a diffusion equation,…”