%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2020-12-16 15:40:22
 % @LastEditTime: 2020-12-16 15:40:30
 
 % I = eye(2,2);
 % PP = u*(F + F'-2*I) + l * trace(F-I) * I;
%--------------------------
function P = PK1_fast_linear(u, l, F)
    m = size(F,2);
    
    FT  = F([1,3,2,4], :);     % 4*m
    trF = F(1,:) + F(4,:) - 2; % 1*m

    I = repmat([1;0;0;1], 1, m); % 4*m

    P = u(:)' .* (F + FT - 2*I) + l(:)' .* trF .* I;
end