function y = ff( x )
%fluctuation function
y=zeros(2,1);
sigma=0;

% y(1)=sigma;
% y(2)=sigma;
%simple additive

% y(1)=sigma*x(1);
% y(2)=sigma*x(2);
%simple multiplicative

sigma=1;
y(1)=0*sigma+0*x(1);
if (x(2)>=0)
y(2)=sigma*sqrt(x(2));
else y(2)=0;
end
%feller logistic
end