编写基于问题的最小二乘法的目标函数
要指定基于问题的最小二乘法的目标函数,请将目标明确写为平方和或表达式范数的平方。通过明确使用最小二乘表示,您可以获得最合适、最有效的求解器问题的方案。例如,
t = randn(10,1); % Data for the example x = optimvar("x",10); obj = sum((x - t).^2); % Explicit sum of squares prob = optimproblem(Objective=obj); % Check to see the default solver solver = solvers(prob)
solver = "lsqlin"
等效地,将目标写为平方范数。
obj2 = norm(x-t)^2; prob2 = optimproblem(Objective=obj2); solver2 = solvers(prob2)
solver2 = "lsqlin"
相反,将目标表示为数学上等价的表达式会产生一个问题,软件会将其解释为一般二次问题。
obj3 = (x - t)'*(x - t); % Equivalent to a sum of squares, % but not interpreted as a sum of squares prob3 = optimproblem(Objective=obj3); solver3 = solvers(prob3)
solver3 = "quadprog"
类似地,将非线性最小二乘写为范数的平方或优化表达式的显式平方和。这个目标是一个明确的平方和。
t = linspace(0,5); % Data for the example A = optimvar("A"); r = optimvar("r"); expr = A*exp(r*t); ydata = 3*exp(-2*t) + 0.1*randn(size(t)); obj4 = sum((expr - ydata).^2); % Explicit sum of squares prob4 = optimproblem(Objective=obj4); solver4 = solvers(prob4)
solver4 = "lsqnonlin"
等效地,将目标写为平方范数。
obj5 = norm(expr - ydata)^2; % norm squared
prob5 = optimproblem(Objective=obj5);
solver5 = solvers(prob5)
solver5 = "lsqnonlin"
软件解释为最小二乘问题的最一般形式是范数的平方,或者是以下形式的表达式 Rn 的总和:
是任何表达式。如果是多维的,则应使用
.^2
对 逐项进行平方。是一个标量数值。
是正标量数值。
您可以除以 而不是乘以 ,这等效于乘以 。
每个表达式 必须计算为标量,而不是多维值。例如,
x = optimvar("x",10,3,4); y = optimvar("y",10,2); t = randn(10,3,4); % Data for example u = randn(10,2); % Data for example a = randn; % Coefficient k = abs(randn(5,1)); % Positive coefficients % Explicit sums of squares: R1 = a + k(1)*sum(k(2)*sum(k(3)*sum((x - t).^2,3))); R2 = k(4)*sum(k(5)*sum((y - u).^2,2)); R3 = 1 + cos(x(1))^2; prob6 = optimproblem(Objective=R1 + R2 + R3); solver6 = solvers(prob6)
solver6 = "lsqnonlin"