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### 45.1 Introduction to celine

Maxima implementation of Sister Celine’s method. Barton Willis wrote this code. It is released under the Creative Commons CC0 license.

Celine’s method is described in Sections 4.1–4.4 of the book "A=B", by Marko Petkovsek, Herbert S. Wilf, and Doron Zeilberger. This book is available at http://www.math.rutgers.edu/~zeilberg/AeqB.pdf

Let f = F(n,k). The function celine returns a set of recursion relations for F of the form

p_0(n) * fff(n,k) + p_1(n) * fff(n+1,k) + ... + p_p(n) * fff(n+p,k+q),

where p_0 through p_p are polynomials. If Maxima is unable to determine that sum(sum(a(i,j) * F(n+i,k+j),i,0,p),j,0,q) / F(n,k) is a rational function of n and k, celine returns the empty set. When f involves parameters (variables other than n or k), celine might make assumptions about these parameters. Using ’put’ with a key of ’proviso,’ Maxima saves these assumptions on the input label.

To use this function, first load the package integer_sequence, opsubst, and to_poly_solve.

Examples:

    (%i1) load("integer_sequence")$
    (%i2) load("opsubst")$
    (%i3) load("to_poly_solve")$
    (%i4) load("celine")$

    (%i5) celine(n!,n,k,1,0);
    (%o5)       {fff(n + 1, k) - n fff(n, k) - fff(n, k)}

Verification that this result is correct:

    (%i1) load("integer_sequence")$
    (%i2) load("opsubst")$
    (%i3) load("to_poly_solve")$
    (%i4) load("celine")$

    (%i5) g1:{fff(n+1,k)-n*fff(n,k)-fff(n,k)};
    (%o5)       {fff(n + 1, k) - n fff(n, k) - fff(n, k)}

    (%i6) ratsimp(minfactorial(first(g1))),fff(n,k) := n!;
    (%o6)                           0

An example with parameters including the test that the result of the example is correct:

    (%i1) load("integer_sequence")$
    (%i2) load("opsubst")$
    (%i3) load("to_poly_solve")$
    (%i4) load("celine")$

    (%i5) e : pochhammer(a,k) * pochhammer(-k,n) / (pochhammer(b,k));
                               (a)  (- k)
                                  k      n
    (%o5)                      -----------
                                  (b)
                                     k

    (%i6) recur : celine(e,n,k,2,1);
    (%o6) {fff(n + 2, k + 1) - fff(n + 2, k) - b fff(n + 1, k + 1)
     + n ((- fff(n + 1, k + 1)) + 2 fff(n + 1, k) - a fff(n, k)
     - fff(n, k)) + a (fff(n + 1, k) - fff(n, k)) + 2 fff(n + 1, k)
        2
     - n  fff(n, k)}

    (%i7) /* Test this result for correctness */
    (%i8) first(%), fff(n,k) := ''(e)$

    (%i9) makefact(makegamma(%))$
    (%o9)                           0

    (%i10) minfactorial(factor(minfactorial(factor(%))));

The proviso data suggests that setting a = b may result in a lower order recursion which is shown by the following example:

    (%i1) load("integer_sequence")$
    (%i2) load("opsubst")$
    (%i3) load("to_poly_solve")$
    (%i4) load("celine")$

    (%i5) e : pochhammer(a,k) * pochhammer(-k,n) / (pochhammer(b,k));
                               (a)  (- k)
                                  k      n
    (%o5)                      -----------
                                  (b)
                                     k

    (%i6) recur : celine(e,n,k,2,1);
    (%o6) {fff(n + 2, k + 1) - fff(n + 2, k) - b fff(n + 1, k + 1)
     + n ((- fff(n + 1, k + 1)) + 2 fff(n + 1, k) - a fff(n, k)
     - fff(n, k)) + a (fff(n + 1, k) - fff(n, k)) + 2 fff(n + 1, k)
        2
     - n  fff(n, k)}

    (%i7) get('%,'proviso);
    (%o7)                         false

    (%i8) celine(subst(b=a,e),n,k,1,1);
    (%o8) {fff(n + 1, k + 1) - fff(n + 1, k) + n fff(n, k)
                                                         + fff(n, k)}

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