Say we discretize some differential equation with the following iteration equation:
$\phi ^{n+1}(x,y)= \phi ^{n-1}(x,y) +f(x,y)\phi ^n$
(if you'd like some more specific example, let me know)
The problem is feeding this equation with two $\phi$ so we can start the iteration. We can assume that any continuous $\phi$ at a given instant is a part of a solution of the original equation, but two subsequent $\phi$ (even if very close) assumes a small evolution that might not be a solution of the original equation. Am I making any sense? To put the question in a more simple way: What is the usual approach of creating suitable initial conditions for 3 steps iteration equations?