`recurrence-guess.gp`

This is a spot of Pari/GP code to guess a linear recurrence from a vector of numbers (or vector of polynomials for guessing with further parameters). The result is pretty printed. For example,

read("recurrence-guess.gp"); recurrence_guess([1, 3, 9, 25, 65, 161, 385, 897, 2049, 4609]); => Recurrence length=3 coefficients v[n-3]* [4, -8, 5] *v[n-1] = v[n] v[n] = v[n-1]* [5, -8, 4] *v[n-3] characteristic polynomial x^3 - 5*x^2 + 8*x - 4 = factors (x - 2)^2 roots 2.00000 x - 1 roots 1.00000 generating function (1 - 2*x + 2*x^2)/(1 - 5*x + 8*x^2 - 4*x^3) = (1 - 2*x + 2*x^2) / ( (1 - x) * (1 - 2*x)^2 ) = partial fractions 1/(1 - x) - 1/(1 - 2*x) + 1/(1 - 2*x)^2 as powers n * 2^n + 1 OEIS %H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5,-8,4). %F a(n) = 5*a(n-1) - 8*a(n-2) + 4*a(n-3).

The guess is found by a simple `matsolve()`

. Linear recurrences
include powers, polynomials, and polynomials times powers. Values given can
themselves be GP polynomials for parameterization or (very) limited symbolic
calculation.

`recurrence-guess.gp`

is
free software (free
as in freedom), published under the terms of the
GNU
General Public License (v3 or higher). Download version
14 here. Requires my
`pol-pfrac.gp`

.

`recurrence-guess.gp`

(51k, and sig)

`recurrence-guess-14.tar.gz`

(51k, and sig)

Just `recurrence-guess.gp`

and
`pol-pfrac.gp`

are
enough to run. The sig files are Gnu PG
ascii armoured signatures generated from my key.

The `tar`

file includes some self-tests, and an
`examples/polmod.gp`

script illustrating linear recurrence
evaluation using `t_POLMOD`

, which is efficient and compact but a
little obscure.

To install so `recurrence_guess()`

is always available
interactively, put `recurrence-guess.gp`

in say your
`~/gp`

directory (which is in the GP
`default(path)`

)
then in file `~/.gprc`

```
````read "recurrence-guess.gp"`

Give a full path (possibly starting `~/`

) if installed somewhere
else.
Similar code can be found in

Bill Allombert points out too that`bestapprPade(Ser(vec))`

gives a generating function. On a long recurrence, sometimes
`lindep()`

seems faster (would intend to use that if so). The nice
output is the tedious part. Of course "nice" is a matter of personal
preference and the output is still quite mechanical.
This page Copyright 2016, 2017, 2018, 2019, 2020, 2021 Kevin Ryde, except for the GPLv3 logo which is Copyright Free Software Foundation and used here in accordance with its terms.

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