The combination of diffusion (Fick 1855) and random walk
(Chandrasekhar 1943)motivated the usual definition of the diffusion
in terms of the mean-square displacement (e.g., Tautz 2012)
κ = (Δx)2
/(2t), which can also be used for a third expression
for Dμμ, namely
Dμμ(μ) =
1
2t
(Δμ(t))2
, (3)
which is again valid if t is high enough. However, the formal
limit t → ∞ is forbidden since |Δμ| cannot exceed a value
of 2. For high enough times, Dμμ will always be dominated by
the 1/t dependence, independent of the choice of the formula.
Therefore, a meaningful, time-independent value for Dμμ can be
obtained if and only if: (i) t is long enough that the initial conditions
become insignificant; (ii) t is short enough that the behavior
of Dμμ is not already dominated by the 1/t proportionality. This
matter is further investigated in the second paper of this series
(Tautz 2013).