ABSTRACT
The function X(t; tˆ, xˆ) : t→ x maps t (time) to x (size). Since g is assumed to be positive, dXdt > 0. Therefore, X is a strictly increasing function and hence has an inverse T (x; tˆ, xˆ) : x → t. The characteristic curve passing through (tˆ, xˆ) is also given by (T (x; tˆ, xˆ), x).
