Kubna metoda - Minimum kod jednodimenzionih promjenjivih

function [xout yout] = kubna(f, fd, sstart, eend, e)
    x = [];
    x(1) = sstart;
    x(2) = eend;

    while true
        % a + bx + cx^2 + dx^3 = f(x)
        % b + 2cx + 3dx^2 = f'(x)
        amat = [
            1 x(1) x(1)^2 x(1)^3;
            1 x(2) x(2)^2 x(2)^3;
            0 1 2*x(1) 3*x(1)^2;
            0 1 2*x(2) 3*x(2)^2;
        ];
        bmat = [f(x(1)); f(x(2)); fd(x(1)); fd(x(2))];
        abcd = linsolve(amat, bmat);
        a = abcd(1);
        b = abcd(2);
        c = abcd(3);
        d = abcd(4);

        % Trazimo x12 za y'(x) = 0
        x12 = [];
        x12(1) = (-2*c + sqrt(4 * c^2 - 4*b*3*d)) / (2*3*d);
        x12(2) = (-2*c - sqrt(4 * c^2 - 4*b*3*d)) / (2*3*d);

        % Trazimo Xopt
        y = @(x)(a + b*x(1) + c*x^2 + d*x^3);
        xopt = x12(2);
        if (y(x12(1)) < y(x12(2)))
            xopt = x12(1);
        end

        % xopt postavljamo kao novu granicu
        if fd(xopt) < 0
            x(1) = xopt;
        else
            x(2) = xopt;
        end

        % Ponvaljamo dok se greska ne bude manje od e
        if abs(y(xopt) - f(xopt)) < e
            break;
        end
    end

    xout = xopt;
    yout = f(xopt);
end