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# 70. solve_rec

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## 70.1 Introduction to solve_rec

`solve_rec` is a package for solving linear recurrences with polynomial coefficients.

A demo is available with `demo(solve_rec)`.

Example:

```(%i1) load("solve_rec")\$
(%i2) solve_rec((n+4)*s[n+2] + s[n+1] - (n+1)*s[n], s[n]);
n
%k  (2 n + 3) (- 1)          %k
1                            2
(%o2)       s  = -------------------- + ---------------
n     (n + 1) (n + 2)      (n + 1) (n + 2)
```

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## 70.2 Functions and Variables for solve_rec

Function: reduce_order (rec, sol, var)

Reduces the order of linear recurrence rec when a particular solution sol is known. The reduced reccurence can be used to get other solutions.

Example:

```(%i3) rec: x[n+2] = x[n+1] + x[n]/n;
x
n
(%o3)               x      = x      + --
n + 2    n + 1   n
(%i4) solve_rec(rec, x[n]);
WARNING: found some hypergeometrical solutions!
(%o4)                    x  = %k  n
n     1
(%i5) reduce_order(rec, n, x[n]);
(%t5)                    x  = n %z
n       n

n - 1
====
\
(%t6)                %z  =  >     %u
n   /        %j
====
%j = 0

(%o6)             (- n - 2) %u     - %u
n + 1     n
(%i6) solve_rec((n+2)*%u[n+1] + %u[n], %u[n]);
n
%k  (- 1)
1
(%o6)                 %u  = ----------
n    (n + 1)!

So the general solution is

n - 1
====        j
\      (- 1)
%k  n  >    -------- + %k  n
2   /     (j + 1)!     1
====
j = 0
```

Option variable: simplify_products

Default value: `true`

If `simplify_products` is `true`, `solve_rec` will try to simplify products in result.

See also: `solve_rec`.

Function: simplify_sum (expr)

Tries to simplify all sums appearing in expr to a closed form.

To use this function first load the `simplify_sum` package with `load(simplify_sum)`.

Example:

```(%i1) load("simplify_sum")\$
(%i2) sum(binom(n+k,k)/2^k, k, 0, n)
+ sum(binom(2*n, 2*k), k, 0, n);
n                            n
====                         ====
\      binomial(n + k, k)    \
(%o2)    >     ------------------ +   >    binomial(2 n, 2 k)
/               k            /
====           2             ====
k = 0                        k = 0
(%i3) simplify_sum(%);
n
4     n
(%o3)                         -- + 2
2
```

Function: solve_rec (eqn, var, [init])

Solves for hypergeometrical solutions to linear recurrence eqn with polynomials coefficient in variable var. Optional arguments init are initial conditions.

`solve_rec` can solve linear recurrences with constant coefficients, finds hypergeometrical solutions to homogeneous linear recurrences with polynomial coefficients, rational solutions to linear recurrences with polynomial coefficients and can solve Ricatti type recurrences.

Note that the running time of the algorithm used to find hypergeometrical solutions is exponential in the degree of the leading and trailing coefficient.

To use this function first load the `solve_rec` package with `load(solve_rec);`.

Example of linear recurrence with constant coefficients:

```(%i2) solve_rec(a[n]=a[n-1]+a[n-2]+n/2^n, a[n]);
n          n
(sqrt(5) - 1)  %k  (- 1)
1           n
(%o2) a  = ------------------------- - ----
n               n                  n
2                5 2
n
(sqrt(5) + 1)  %k
2    2
+ ------------------ - ----
n              n
2            5 2
```

Example of linear recurrence with polynomial coefficients:

```(%i7) 2*x*(x+1)*y[x] - (x^2+3*x-2)*y[x+1] + (x-1)*y[x+2];
2
(%o7) (x - 1) y      - (x  + 3 x - 2) y      + 2 x (x + 1) y
x + 2                   x + 1                x
(%i8) solve_rec(%, y[x], y=1, y=3);
x
3 2    x!
(%o9)                 y  = ---- - --
x    4     2
```

Example of Ricatti type recurrence:

```(%i2) x*y[x+1]*y[x] - y[x+1]/(x+2) + y[x]/(x-1) = 0;
y         y
x + 1     x
(%o2)         x y  y      - ------ + ----- = 0
x  x + 1   x + 2    x - 1
(%i3) solve_rec(%, y[x], y=5)\$
(%i4) ratsimp(minfactorial(factcomb(%)));
3
30 x  - 30 x
(%o4) y  = - -------------------------------------------------
x        6      5       4       3       2
5 x  - 3 x  - 25 x  + 15 x  + 20 x  - 12 x - 1584
```

See also: `solve_rec_rat`, `simplify_products`, and `product_use_gamma`.

Function: solve_rec_rat (eqn, var, [init])

Solves for rational solutions to linear recurrences. See solve_rec for description of arguments.

To use this function first load the `solve_rec` package with `load(solve_rec);`.

Example:

```(%i1) (x+4)*a[x+3] + (x+3)*a[x+2] - x*a[x+1] + (x^2-1)*a[x];
(%o1)  (x + 4) a      + (x + 3) a      - x a
x + 3            x + 2      x + 1
2
+ (x  - 1) a
x
(%i2) solve_rec_rat(% = (x+2)/(x+1), a[x]);
1
(%o2)      a  = ---------------
x   (x - 1) (x + 1)
```

See also: `solve_rec`.

Option variable: product_use_gamma

Default value: `true`

When simplifying products, `solve_rec` introduces gamma function into the expression if `product_use_gamma` is `true`.

See also: `simplify_products`, `solve_rec`.

Function: summand_to_rec (summand, k, n)
Function: summand_to_rec (summand, [k, lo, hi], n)

Returns the recurrence sattisfied by the sum

```     hi
====
\
>     summand
/
====
k = lo
```

where summand is hypergeometrical in k and n. If lo and hi are omited, they are assumed to be `lo = -inf` and `hi = inf`.

To use this function first load the `simplify_sum` package with `load(simplify_sum)`.

Example:

```(%i1) load("simplify_sum")\$
(%i2) summand: binom(n,k);
(%o2)                           binomial(n, k)
(%i3) summand_to_rec(summand,k,n);
(%o3)                      2 sm  - sm      = 0
n     n + 1
(%i7) summand: binom(n, k)/(k+1);
binomial(n, k)
(%o7)                           --------------
k + 1
(%i8) summand_to_rec(summand, [k, 0, n], n);
(%o8)               2 (n + 1) sm  - (n + 2) sm      = - 1
n             n + 1
```

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