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Maxima Code for Lagrange Interpolation

The maxima program is free and open-source, like Octave for matlab:5.3
  (%i1) lagrange(N, n) :=
           product(if equal(k,n) then 1
                  else (D-k)/(n-k), k, 0, N);

  (%o1) lagrange(N, n) := product(if equal(k, n) then 1

                                       D - k
                                  else -----, k, 0, N)
                                       n - k
Usage examples in maxima:
  (%i2) lagrange(1,0);
  (%o2)                             1 - D

  (%i3) lagrange(1,1);
  (%o3)                               D

  (%i4) lagrange(4,0);
                           (1 - D) (D - 4) (D - 3) (D - 2)
  (%o4)                 - -------------------------------
                                         24
  (%i5) ratsimp(lagrange(4,0));
                           4       3       2
                          D  - 10 D  + 35 D  - 50 D + 24
  (%o5)                  ------------------------------
                                      24

  (%i6) expand(lagrange(4,0));
                            4      3       2
                           D    5 D    35 D    25 D
  (%o6)                    -- - ---- + ----- - ---- + 1
                           24    12     24      12

  (%i7) expand(lagrange(4,0)), numer;
                           4                     3
  (%o7) 0.041666666666667 D  - 0.41666666666667 D
                           2
      + 1.458333333333333 D  - 2.083333333333333 D + 1.0



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Matlab Code for Lagrange Interpolation