eigval.m

function [evalues, repeats] = eigval(A)

% eigval  Eigenvalues and their algebraic multiplicity.
%
% evalues = eigval(A) returns the distinct eigenvalues of A,
% with duplicates removed and sorted in decreasing order.
%
% [evalues, repeats] = eigval(A) also returns the row vector
% repeats that gives the multiplicity of each eigenvalue.
% The sum of the multiplicities is n.
%
% Examples: Let A = eye(n) and B = diag([3 4]).
% For A, evalues is 1 and repeats is n.
% For B, evalues is [4; 3]  and repeats is [1 1].

tol = sqrt(eps);
lambda = sort(eig(A));
lambda = round(lambda/tol) * tol;
%
% lambda gives all n eigenvalues (repetitions included).
%
evalues = unique(lambda);
evalues = flipud(evalues);
n = length(lambda);
d = length(evalues);
A = ones(n, 1) * evalues';
B = lambda * ones(1, d);
MATCH = abs(A-B) <= tol;
%
% MATCH is an n by d zero matrix except
% MATCH(i,j) = 1 when lambda(i) = evalues(j).
% Summing the columns gives the row vector repeats.
%
repeats = sum(MATCH);