Well, sorry for the code... :/

Just copied my work sheet here, I thought it would work...

And thank you for your answer, it does now end the program... :) I just need to understand why it doesn't give me the curve I want to.

Here is the text version :

with(linalg):

with(DEtools):

SecTer := crossprod(``(B[x](t), B[y](t), B[z](t)), ``(diff(x(t), t), diff(y(t), t), 0));

ProjX := diff(x(t), t, t) = -q*innerprod(SecTer, ``(1, 0, 0))/m;

ProjY := diff(y(t), t, t) = -q*innerprod(SecTer, ``(0, 1, 0))/m;

B[x] := t->0;

B[y] := t->0;

B[z] := t-> 1 + diff(y(t), t);

q := 1.6*10^(-19); m := 9, 10^(-31);

DEplot([ProjX, ProjY], [x(t), y(t)], t = 0 .. 40, [[x(0) = 0, y(0) = 0, (D(x))(0) = 0, (D(y))(0) = 1]], arrows = medium, scene = [x(t), y(t)])

EDIT : It now tells me that DEtools/convertsys receives 3 arguments while it only asks for two... Which is quite strange since I managed once to plot what I wanted to...

My mistake, I had put a coma instead of a dot in a value.