Lagranges

function LG = lagranges(x, y, a)
    n = length(x);
    LG = 0;
    for i = 1 : n
        w = 1;
        for j = 1 : n
            if i ~= j
                w = (w * (a - x(j))) / (x(i) - x(j));
            end
        end
        LG = LG + w * y(i);
    end