% deriv = chargexbderi(t,yac) Ableitung Teilchen 2D
%   in konstantem Magnetfeld mit E-feld in y-Richtung
function deriv = chargexbderi(t,yac)
B=-1/10;  deriv = zeros(4,1); E=0.02;
deriv(1) = yac(2);  deriv(2) =  yac(4)*B ;
deriv(3) = yac(4);  deriv(4) = -yac(2)*B +E ;
