def rsolve_hyper(coeffs, f, n, **hints):
"""Given linear recurrence operator L of order 'k' with polynomial
coefficients and inhomogeneous equation Ly = f we seek for all
hypergeometric solutions over field K of characteristic zero.
The inhomogeneous part can be either hypergeometric or a sum
of a fixed number of pairwise dissimilar hypergeometric terms.
The algorithm performs three basic steps:
(1) Group together similar hypergeometric terms in the
inhomogeneous part of Ly = f, and find particular
solution using Abramov's algorithm.
(2) Compute generating set of L and find basis in it,
so that all solutions are linearly independent.
(3) Form final solution with the number of arbitrary
constants equal to dimension of basis of L.
Term a(n) is hypergeometric if it is annihilated by first order
linear difference equations with polynomial coefficients or, in
simpler words, if consecutive term ratio is a rational function.
The output of this procedure is a linear combination of fixed
number of hypergeometric terms. However the underlying method
can generate larger class of solutions - D'Alembertian terms.
Note also that this method not only computes the kernel of the
inhomogeneous equation, but also reduces in to a basis so that
solutions generated by this procedure are linearly independent
For more information on the implemented algorithm refer to:
[1] M. Petkovsek, Hypergeometric solutions of linear recurrences
with polynomial coefficients, J. Symbolic Computation,
14 (1992), 243-264.
[2] M. Petkovsek, H. S. Wilf, D. Zeilberger, A = B, 1996.
"""
coeffs = map(sympify, coeffs)
f = sympify(f)
r, kernel = len(coeffs)-1, []
if not f.is_zero:
if f.is_Add:
similar = {}
for g in f.expand().args:
if not g.is_hypergeometric(n):
return None
for h in similar.iterkeys():
if hypersimilar(g, h, n):
similar[h] += g
break
else:
similar[g] = S.Zero
inhomogeneous = []
for g, h in similar.iteritems():
inhomogeneous.append(g+h)
elif f.is_hypergeometric(n):
inhomogeneous = [f]
else:
return None
for i, g in enumerate(inhomogeneous):
coeff, polys = S.One, coeffs[:]
denoms = [ S.One ] * (r+1)
s = hypersimp(g, n)
for j in xrange(1, r+1):
coeff *= s.subs(n, n+j-1)
p, q = coeff.as_numer_denom()
polys[j] *= p
denoms[j] = q
for j in xrange(0, r+1):
polys[j] *= Mul(*(denoms[:j] + denoms[j+1:]))
R = rsolve_poly(polys, Mul(*denoms), n)
if not (R is None or R is S.Zero):
inhomogeneous[i] *= R
else:
return None
result = Add(*inhomogeneous)
else:
result = S.Zero
Z = Symbol('Z', dummy=True)
p, q = coeffs[0], coeffs[r].subs(n, n-r+1)
p_factors = [ z for z in roots(p, n).iterkeys() ]
q_factors = [ z for z in roots(q, n).iterkeys() ]
factors = [ (S.One, S.One) ]
for p in p_factors:
for q in q_factors:
if p.is_integer and q.is_integer and p <= q:
continue
else:
factors += [(n-p, n-q)]
p = [ (n-p, S.One) for p in p_factors ]
q = [ (S.One, n-q) for q in q_factors ]
factors = p + factors + q
for A, B in factors:
polys, degrees = [], []
D = A*B.subs(n, n+r-1)
for i in xrange(0, r+1):
a = Mul(*[ A.subs(n, n+j) for j in xrange(0, i) ])
b = Mul(*[ B.subs(n, n+j) for j in xrange(i, r) ])
poly = exquo(coeffs[i]*a*b, D, n)
polys.append(poly.as_poly(n))
if not poly.is_zero:
degrees.append(polys[i].degree())
d, poly = max(degrees), S.Zero
for i in xrange(0, r+1):
coeff = polys[i].nth(d)
if coeff is not S.Zero:
poly += coeff * Z**i
for z in roots(poly, Z).iterkeys():
if not z.is_real or z.is_zero:
continue
C = rsolve_poly([ polys[i]*z**i for i in xrange(r+1) ], 0, n)
if C is not None and C is not S.Zero:
ratio = z * A * C.subs(n, n + 1) / B / C
K = product(simplify(ratio), (n, 0, n-1))
if casoratian(kernel+[K], n) != 0:
kernel.append(K)
symbols = [ Symbol('C'+str(i)) for i in xrange(len(kernel)) ]
for C, ker in zip(symbols, kernel):
result += C * ker
if hints.get('symbols', False):
return (result, symbols)
else:
return result
def rsolve_hyper(coeffs, f, n, **hints):
"""
Given linear recurrence operator `\operatorname{L}` of order `k`
with polynomial coefficients and inhomogeneous equation
`\operatorname{L} y = f` we seek for all hypergeometric solutions
over field `K` of characteristic zero.
The inhomogeneous part can be either hypergeometric or a sum
of a fixed number of pairwise dissimilar hypergeometric terms.
The algorithm performs three basic steps:
(1) Group together similar hypergeometric terms in the
inhomogeneous part of `\operatorname{L} y = f`, and find
particular solution using Abramov's algorithm.
(2) Compute generating set of `\operatorname{L}` and find basis
in it, so that all solutions are linearly independent.
(3) Form final solution with the number of arbitrary
constants equal to dimension of basis of `\operatorname{L}`.
Term `a(n)` is hypergeometric if it is annihilated by first order
linear difference equations with polynomial coefficients or, in
simpler words, if consecutive term ratio is a rational function.
The output of this procedure is a linear combination of fixed
number of hypergeometric terms. However the underlying method
can generate larger class of solutions - D'Alembertian terms.
Note also that this method not only computes the kernel of the
inhomogeneous equation, but also reduces in to a basis so that
solutions generated by this procedure are linearly independent
Examples
========
>>> from sympy.solvers import rsolve_hyper
>>> from sympy.abc import x
>>> rsolve_hyper([-1, -1, 1], 0, x)
C0*(1/2 + sqrt(5)/2)**x + C1*(-sqrt(5)/2 + 1/2)**x
>>> rsolve_hyper([-1, 1], 1 + x, x)
C0 + x*(x + 1)/2
References
==========
.. [1] M. Petkovsek, Hypergeometric solutions of linear recurrences
with polynomial coefficients, J. Symbolic Computation,
14 (1992), 243-264.
.. [2] M. Petkovsek, H. S. Wilf, D. Zeilberger, A = B, 1996.
"""
coeffs = map(sympify, coeffs)
f = sympify(f)
r, kernel, symbols = len(coeffs) - 1, [], set()
if not f.is_zero:
if f.is_Add:
similar = {}
for g in f.expand().args:
if not g.is_hypergeometric(n):
return None
for h in similar.iterkeys():
if hypersimilar(g, h, n):
similar[h] += g
break
else:
similar[g] = S.Zero
inhomogeneous = []
for g, h in similar.iteritems():
inhomogeneous.append(g + h)
elif f.is_hypergeometric(n):
inhomogeneous = [f]
else:
return None
for i, g in enumerate(inhomogeneous):
coeff, polys = S.One, coeffs[:]
denoms = [ S.One ] * (r + 1)
s = hypersimp(g, n)
for j in xrange(1, r + 1):
coeff *= s.subs(n, n + j - 1)
p, q = coeff.as_numer_denom()
polys[j] *= p
denoms[j] = q
for j in xrange(0, r + 1):
polys[j] *= Mul(*(denoms[:j] + denoms[j + 1:]))
R = rsolve_poly(polys, Mul(*denoms), n)
if not (R is None or R is S.Zero):
inhomogeneous[i] *= R
else:
return None
result = Add(*inhomogeneous)
else:
result = S.Zero
Z = Dummy('Z')
p, q = coeffs[0], coeffs[r].subs(n, n - r + 1)
p_factors = [ z for z in roots(p, n).iterkeys() ]
q_factors = [ z for z in roots(q, n).iterkeys() ]
factors = [ (S.One, S.One) ]
for p in p_factors:
for q in q_factors:
if p.is_integer and q.is_integer and p <= q:
continue
else:
factors += [(n - p, n - q)]
p = [ (n - p, S.One) for p in p_factors ]
q = [ (S.One, n - q) for q in q_factors ]
factors = p + factors + q
for A, B in factors:
polys, degrees = [], []
D = A*B.subs(n, n + r - 1)
for i in xrange(0, r + 1):
a = Mul(*[ A.subs(n, n + j) for j in xrange(0, i) ])
b = Mul(*[ B.subs(n, n + j) for j in xrange(i, r) ])
poly = quo(coeffs[i]*a*b, D, n)
polys.append(poly.as_poly(n))
if not poly.is_zero:
degrees.append(polys[i].degree())
d, poly = max(degrees), S.Zero
for i in xrange(0, r + 1):
coeff = polys[i].nth(d)
if coeff is not S.Zero:
poly += coeff * Z**i
for z in roots(poly, Z).iterkeys():
if z.is_zero:
continue
(C, s) = rsolve_poly([ polys[i]*z**i for i in xrange(r + 1) ], 0, n, symbols=True)
if C is not None and C is not S.Zero:
symbols |= set(s)
ratio = z * A * C.subs(n, n + 1) / B / C
ratio = simplify(ratio)
# If there is a nonnegative root in the denominator of the ratio,
# this indicates that the term y(n_root) is zero, and one should
# start the product with the term y(n_root + 1).
n0 = 0
for n_root in roots(ratio.as_numer_denom()[1], n).keys():
if (n0 < (n_root + 1)) is True:
n0 = n_root + 1
K = product(ratio, (n, n0, n - 1))
if K.has(factorial, FallingFactorial, RisingFactorial):
K = simplify(K)
if casoratian(kernel + [K], n, zero=False) != 0:
kernel.append(K)
kernel.sort(key=default_sort_key)
sk = zip(numbered_symbols('C'), kernel)
if sk:
for C, ker in sk:
result += C * ker
else:
return None
if hints.get('symbols', False):
symbols |= set([s for s, k in sk])
return (result, list(symbols))
else:
return result