I can not figure out what happened here. I must have missed something maybe, but I suspect that something is wrong with Jacob() function. Look at this picture please:
Radovan, I've improved Jacob(..) function - now it will show an error message instead of zeros. The thing is that the example you've shown on screenshot is incorrect - you have defined z variable as a 3 components vector of numbers. Jacob(..) function cannot operate with such input data.
I thought there must be something like you explained. It would be quite enough to issue an error message when the arguments are inappropriate. The worst thing is when the faulty results are still returned.
Regards, Radovan
When Sisyphus climbed to the top of a hill, they said: "Wrong boulder!"