function y = gpop(x,t,pop)
% 2D function
y1 = sum((x(1,1)*exp(x(2,1)*t)-pop).*exp(x(2,1)*t));
y2 = sum((x(1,1)*exp(x(2,1)*t)-pop).*(x(1,1)*t)...
    .*exp(x(2,1)*t));
y = [y1;y2];
end

