Wengert's Numerical Method for Partial Derivatives,
Orbit Determination and Quasilinearization
In a recent article in the Communications
of the ACM, R. Wengert suggested a technique for 
machine evaluation of the partial derivatives of a function
given in analytical form.  In solving non-linear 
boundary-value problems using quasilinearization many
partial derivatives must be formed analytically 
and then evaluated numerically.  Wengert's method appears
very attractive from the programming viewpoint 
equations which might not otherwise be undertaken.
CACM April, 1965
Bellman, R. E.
Kagiwada, H.
Kalaba, R. E.
