> The Jacobian determinant of your function f(a,b,c,d) is 0 at [1,1,1,1].
Nevertheless, I suspect that there is an error somewhere:
Unable to compute the LU factorization
#0: solve_by_lu(
Maxima encountered a Lisp error:
Error in MERROR [or a callee]: NIL is not of type CHARACTER.
The user should get a clear message, not a Lisp error.
I'd guess this is a bug in the LU code, but the mnewton code could use
improvement.
Maybe the bug is related to the (ill-advised) way that the mnewton code
uses an
undeclared array h.
--Barton