function p=trislice(a,b,lam) 
% TRISLICE - Spectrum slicing for a tridiagonal matrix 
% Syntax: p=trislice(a,b,lam); 
% 
% Input:  a - vector of length N, containing the diagonal elements of A. 
%         b - vector of length N-1, containing the off-diagonal elements of A. 
%       lam - eigenvalue estimate(s). lam may be a vector. 
% 
% Output: p - number of eigenvalues of A greater than lam. If lam 
%              is a vector, pi has n elements, which correspond to the  
%              elements of lam.  
% 
 
a=a(:); 
lam=lam(:); 
N=length(a); 
n=length(lam); 
p1=zeros(n,N); 
won=ones(n,1); 
b=b(:).^2; 
 
p1(:,1)=won*a(1)-lam; 
 
z=find(p1(:,1)==0); 
%p1(z,1)=ones(length(z),1)*eps; % replace any zeros with eps. 
 
for i=2:N 
  p1(:,i)=won*a(i)-lam-won*b(i-1)./p1(:,i-1); 
%  z=find(p1(:,i)==0); 
%  p1(z,i)=ones(length(z),i)*eps; % replace any zeros with eps. 
end 
 
p=zeros(1,n); 
for i=1:n 
  p(i)=length(find(p1(i,:)>0)); 
end