function s=lagrange(x,y,k)
  n=length(x)
  s=0
  for i=1:n
    p=x
    p(i)=[]
    t=1
    for j=1:n-1
      t=t*(k-p(j))/(x(i)-p(j))
    endfor
    s=s+y(i)*t
    endfor
endfunction