function X=central(pf, n) 
% central(pf, n)  calculates the nth central moment of a pdf or pmf 
if ispf(pf)==0 
   error('invalid probability function') 
end 
p=pvals(pf); 
x=xvals(pf); 
u=mmean(pf); 
g=((x-u).^n).*p; 
if isdiscrt(pf)==1 
   X=sum(g); 
else 
   X=trapz(x,g); 
end