  The persistence exponent $\theta_o$ for the simple diffusion equation
${\phi}_t({\it x},t) = \triangle \phi (x,t)$ , with random Gaussian initial
condition {\color{red},} has been calculated exactly using a method known as
selective averaging. The probability that the value of the field $\phi$ at a
specified spatial coordinate remains positive throughout for a certain time $t$
behaves as $t^{-\theta_o}$ for asymptotically large time $t$. The value of
$\theta_o$, calculated here for any integer dimension $d$, is $\theta_o =
\frac{d}{4}$ for $d\leq 4$ and $1$ otherwise. This exact theoretical result is
being reported possibly for the first time and is not in agreement with the
accepted values $ \theta_o = 0.12, 0.18,0.23$ for $d=1,2,3$ respectively.
