function AF = agefact(age, D)
%AGEFACT(age) Calculates Ijspeert's age factor (AF) given age in years
%	and doubling age (D). If not specified D is set at the default 70 years,
% 	based on Ijspeert's equations.
% 
switch nargin
	case {0}
		age = 0;
		D = 70;
	case {1}
		D = 70;
	case {2}
		
	otherwise
		error 'Error: please specify only age and doubling age'
end

if D==0 
	error 'D cannot be zero' %error checking to avoid dividing by zero
end


AF = 1 + (age/D)^4; %Ijspeert actual AF equation