function pdf=dferlan(lamda, r, samples) 
% dferlan(lamda ,r, samples)  Generates an Erlang pdf with parameters lamda 
%    and r.  samples sets the size of the numerical representation, and is 
%    optional.  The default sample size is 1000. 
if nargin==2 
   samples=1000; 
end 
if lamda<=0 
   error('invalid value for lamda') 
end 
if r<=0 
   error('invalid value for r') 
end 
V=sqrt(r)/lamda; 
X=5*V; 
while (lamda^r)*(X^(r-1))*exp(-lamda*X)/fact(r-1) > 1e-6 
   X=X+V; 
end 
x=[0:X/samples:X]; 
p=(lamda.^r).*(x.^(r-1)).*exp(-lamda.*x)./fact(r-1); 
pdf=[0 1; x' p']; 
if ispf(pdf)==0 
   p=p./ttlprob(pdf); 
   pdf=[0 1; x' p']; 
end