def guess(l, all=False, evaluate=True, niter=2, variables=None):
"""
This function is adapted from the Rate.m package for Mathematica
written by Christian Krattenthaler.
It tries to guess a formula from a given sequence of rational numbers.
In order to speed up the process, the 'all' variable is set to False by
default, stopping the computation as some results are returned during an
iteration; the variable can be set to True if more iterations are needed
(other formulas may be found; however they may be equivalent to the first
ones).
Another option is the 'evaluate' variable (default is True); setting it
to False will leave the involved products unevaluated.
By default, the number of iterations is set to 2 but a greater value (up
to len(l)-1) can be specified with the optional 'niter' variable.
More and more convoluted results are found when the order of the
iteration gets higher:
* first iteration returns polynomial or rational functions;
* second iteration returns products of rising factorials and their
inverses;
* third iteration returns products of products of rising factorials
and their inverses;
* etc.
The returned formulas contain symbols i0, i1, i2, ... where the main
variables is i0 (and auxiliary variables are i1, i2, ...). A list of
other symbols can be provided in the 'variables' option; the length of
the least should be the value of 'niter' (more is acceptable but only
the first symbols will be used); in this case, the main variable will be
the first symbol in the list.
Examples
========
>>> from sympy.concrete.guess import guess
>>> guess([1,2,6,24,120], evaluate=False)
[Product(i1 + 1, (i1, 1, i0 - 1))]
>>> from sympy import symbols
>>> r = guess([1,2,7,42,429,7436,218348,10850216], niter=4)
>>> i0 = symbols("i0")
>>> [r[0].subs(i0,n).doit() for n in range(1,10)]
[1, 2, 7, 42, 429, 7436, 218348, 10850216, 911835460]
"""
if any(a == 0 for a in l[:-1]):
return []
N = len(l)
niter = min(N - 1, niter)
myprod = product if evaluate else Product
g = []
res = []
if variables is None:
symb = symbols('i:' + str(niter))
else:
symb = variables
for k, s in enumerate(symb):
g.append(l)
n, r = len(l), []
for i in range(n - 2 - 1, -1, -1):
ri = rinterp(enumerate(g[k][:-1], start=1), i, X=s)
if ((denom(ri).subs({s: n}) != 0)
and (ri.subs({s: n}) - g[k][-1] == 0) and ri not in r):
r.append(ri)
if r:
for i in range(k - 1, -1, -1):
r = list(
map(
lambda v: g[i][0] * myprod(v, (symb[i + 1], 1, symb[i]
- 1)), r))
if not all: return r
res += r
l = [Rational(l[i + 1], l[i]) for i in range(N - k - 1)]
return res
def guess(l, all=False, evaluate=True, niter=2, variables=None):
"""
This function is adapted from the Rate.m package for Mathematica
written by Christian Krattenthaler.
It tries to guess a formula from a given sequence of rational numbers.
In order to speed up the process, the 'all' variable is set to False by
default, stopping the computation as some results are returned during an
iteration; the variable can be set to True if more iterations are needed
(other formulas may be found; however they may be equivalent to the first
ones).
Another option is the 'evaluate' variable (default is True); setting it
to False will leave the involved products unevaluated.
By default, the number of iterations is set to 2 but a greater value (up
to len(l)-1) can be specified with the optional 'niter' variable.
More and more convoluted results are found when the order of the
iteration gets higher:
* first iteration returns polynomial or rational functions;
* second iteration returns products of rising factorials and their
inverses;
* third iteration returns products of products of rising factorials
and their inverses;
* etc.
The returned formulas contain symbols i0, i1, i2, ... where the main
variables is i0 (and auxiliary variables are i1, i2, ...). A list of
other symbols can be provided in the 'variables' option; the length of
the least should be the value of 'niter' (more is acceptable but only
the first symbols will be used); in this case, the main variable will be
the first symbol in the list.
>>> from sympy.concrete.guess import guess
>>> guess([1,2,6,24,120], evaluate=False)
[Product(i1 + 1, (i1, 1, i0 - 1))]
>>> from sympy import symbols
>>> r = guess([1,2,7,42,429,7436,218348,10850216], niter=4)
>>> i0 = symbols("i0")
>>> [r[0].subs(i0,n).doit() for n in range(1,10)]
[1, 2, 7, 42, 429, 7436, 218348, 10850216, 911835460]
"""
if any(a==0 for a in l[:-1]):
return []
N = len(l)
niter = min(N-1, niter)
myprod = product if evaluate else Product
g = []
res = []
if variables == None:
symb = symbols('i:'+str(niter))
else:
symb = variables
for k, s in enumerate(symb):
g.append(l)
n, r = len(l), []
for i in range(n-2-1, -1, -1):
ri = rinterp(enumerate(g[k][:-1], start=1), i, X=s)
if ((denom(ri).subs({s:n}) != 0)
and (ri.subs({s:n}) - g[k][-1] == 0)
and ri not in r):
r.append(ri)
if r:
for i in range(k-1, -1, -1):
r = list(map(lambda v: g[i][0]
* myprod(v, (symb[i+1], 1, symb[i]-1)), r))
if not all: return r
res += r
l = [Rational(l[i+1], l[i]) for i in range(N-k-1)]
return res