## 103 Package zeilberger ¶

### 103.1 Introduction to zeilberger ¶

`zeilberger` is an implementation of Zeilberger’s algorithm for definite hypergeometric summation, and also Gosper’s algorithm for indefinite hypergeometric summation.

`zeilberger` makes use of the "filtering" optimization method developed by Axel Riese.

`zeilberger` was developed by Fabrizio Caruso.

`load ("zeilberger")` loads this package.

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#### 103.1.1 The indefinite summation problem ¶

`zeilberger` implements Gosper’s algorithm for indefinite hypergeometric summation. Given a hypergeometric term \(F_k\) in \(k\) we want to find its hypergeometric anti-difference, that is, a hypergeometric term \(f_k\) such that

    \[F_k = f_{k+1} - f_k.
    \]

#### 103.1.2 The definite summation problem ¶

`zeilberger` implements Zeilberger’s algorithm for definite hypergeometric summation. Given a proper hypergeometric term (in \(n\) and \(k\)) \(F_{n,k}\) and a positive integer \(d\) we want to find a \(d\)-th order linear recurrence with polynomial coefficients (in \(n\)) for \(F_{n,k}\) and a rational function \(R\) in \(n\) and \(k\) such that

    \[a_0 \, F_{n,k} + \ldots + a_d \, F_{n+d,k} = \Delta_K \left(R\left(n,k\right) F_{n,k}\right),
    \]

where \(\Delta_k\) is the \(k\)-forward difference operator, i.e., \(\Delta_k \left(t_k\right) \equiv t_{k+1} - t_k.\)

#### 103.1.3 Verbosity levels ¶

There are also verbose versions of the commands which are called by adding one of the following prefixes:

`Summary`

  Just a summary at the end is shown

`Verbose`

  Some information in the intermediate steps

`VeryVerbose`

  More information

`Extra`

  Even more information including information on the linear system in Zeilberger’s algorithm

For example:
`GosperVerbose`, `parGosperVeryVerbose`, `ZeilbergerExtra`, `AntiDifferenceSummary`.

### 103.2 Functions and Variables for zeilberger ¶

Function: AntiDifference (\(F_k\), k) ¶

  Returns the hypergeometric anti-difference of \(F_k\), if it exists.
  Otherwise `AntiDifference` returns `no_hyp_antidifference`.

  Categories: Package zeilberger ·

Function: Gosper (\(F_k\), k) ¶

  Returns the rational certificate \(R(k)\) for \(F_k\), that is, a rational function such that \(F_k = R\left(k+1\right) \, F_{k+1} - R\left(k\right) \, F_k,\) if it exists. Otherwise, `Gosper` returns `no_hyp_sol`.

  Categories: Package zeilberger ·

Function: GosperSum (\(F_k\), k, a, b) ¶

  Returns the summation of \(F_k\) from \(\mathit{k} = \mathit{a}\) to \(\mathit{k} = \mathit{b}\) if \(F_k\) has a hypergeometric anti-difference. Otherwise, `GosperSum` returns `nongosper_summable`.

  Examples:

      (%i1) load ("zeilberger")$

      (%i2) GosperSum ((-1)^k*k / (4*k^2 - 1), k, 1, n);
                                 n + 1      3
                            (- 1)      (n + -)
                                            2    1
      (%o2)               - ------------------ - -
                                        2        4
                            2 (4 (n + 1)  - 1)

      (%i3) GosperSum (1 / (4*k^2 - 1), k, 1, n);
                                      3
                                - n - -
                                      2       1
      (%o3)                  -------------- + -
                                      2       2
                             4 (n + 1)  - 1

      (%i4) GosperSum (x^k, k, 1, n);
                                n + 1
                               x          x
      (%o4)                    ------ - -----
                               x - 1    x - 1

      (%i5) GosperSum ((-1)^k*a! / (k!*(a - k)!), k, 1, n);
                           n + 1
                      (- 1)      a! (n + 1)         a!
      (%o5)       - ------------------------- - ----------
                    a (- n + a - 1)! (n + 1)!   (a - 1)! a

      (%i6) GosperSum (k*k!, k, 1, n);
      (%o6)                     (n + 1)! - 1

      (%i7) GosperSum ((k + 1)*k! / (k + 1)!, k, 1, n);
                        (n + 1) (n + 2) (n + 1)!
      (%o7)             ------------------------ - 1
                                (n + 2)!

      (%i8) GosperSum (1 / ((a - k)!*k!), k, 1, n);
      (%o8)                  NON_GOSPER_SUMMABLE

  Categories: Package zeilberger ·

Function: parGosper (\(F_(n,k)\), k, n, d) ¶

  Attempts to find a d-th order recurrence for \(F_(n,k)\).

  The algorithm yields a sequence \([s_1, s_2, ..., s_m]\) of solutions. Each solution has the form

  \([R(n, k), [a_0, a_1, ..., a_d]].\)

  `parGosper` returns `[]` if it fails to find a recurrence.

  Categories: Package zeilberger ·

Function: Zeilberger (\(F_(n,k)\), k, n) ¶

  Attempts to compute the indefinite hypergeometric summation of \(F_(n,k)\).

  `Zeilberger` first invokes `Gosper`, and if that fails to find a solution, then invokes `parGosper` with order 1, 2, 3, ..., up to `MAX_ORD`. If Zeilberger finds a solution before reaching `MAX_ORD`, it stops and returns the solution.

  The algorithms yields a sequence \([s_1, s_2, ..., s_m]\) of solutions. Each solution has the form

  \([R(n,k), [a_0, a_1, ..., a_d]].\)

  `Zeilberger` returns `[]` if it fails to find a solution.

  `Zeilberger` invokes `Gosper` only if `Gosper_in_Zeilberger` is `true`.

  Categories: Package zeilberger ·

### 103.3 General global variables ¶

Global variable: MAX_ORD ¶

  Default value: 5

  `MAX_ORD` is the maximum recurrence order attempted by `Zeilberger`.

  Categories: Package zeilberger ·

Global variable: simplified_output ¶

  Default value: `false`

  When `simplified_output` is `true`, functions in the `zeilberger` package attempt further simplification of the solution.

  Categories: Package zeilberger ·

Global variable: linear_solver ¶

  Default value: `linsolve`

  `linear_solver` names the solver which is used to solve the system of equations in Zeilberger’s algorithm.

  Categories: Package zeilberger ·

Global variable: warnings ¶

  Default value: `true`

  When `warnings` is `true`, functions in the `zeilberger` package print warning messages during execution.

  Categories: Package zeilberger ·

Global variable: Gosper_in_Zeilberger ¶

  Default value: `true`

  When `Gosper_in_Zeilberger` is `true`, the `Zeilberger` function calls `Gosper` before calling `parGosper`. Otherwise, `Zeilberger` goes immediately to `parGosper`.

  Categories: Package zeilberger ·

Global variable: trivial_solutions ¶

  Default value: `true`

  When `trivial_solutions` is `true`, `Zeilberger` returns solutions which have certificate equal to zero, or all coefficients equal to zero.

  Categories: Package zeilberger ·

### 103.4 Variables related to the modular test ¶

Global variable: mod_test ¶

  Default value: `false`

  When `mod_test` is `true`, `parGosper` executes a modular test for discarding systems with no solutions.

  Categories: Package zeilberger ·

Global variable: modular_linear_solver ¶

  Default value: `linsolve`

  `modular_linear_solver` names the linear solver used by the modular test in `parGosper`.

  Categories: Package zeilberger ·

Global variable: ev_point ¶

  Default value: `big_primes[10]`

  `ev_point` is the value at which the variable n is evaluated when executing the modular test in `parGosper`.

  Categories: Package zeilberger ·

Global variable: mod_big_prime ¶

  Default value: `big_primes[1]`

  `mod_big_prime` is the modulus used by the modular test in `parGosper`.

  Categories: Package zeilberger ·

Global variable: mod_threshold ¶

  Default value: 4

  `mod_threshold` is the greatest order for which the modular test in `parGosper` is attempted.

  Categories: Package zeilberger ·

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