Source code for sympy.simplify.sqrtdenest

from sympy.functions import sqrt, sign, root
from sympy.core import S, Wild, sympify, Mul, Add, Expr
from sympy.core.function import expand_multinomial, expand_mul
from sympy.core.symbol import Dummy
from sympy.polys import Poly, PolynomialError
from sympy.core.function import count_ops
from sympy.utilities import default_sort_key


def _mexpand(expr):
    return expand_mul(expand_multinomial(expr))


def is_sqrt(expr):
    """Return True if expr is a sqrt, otherwise False."""

    return expr.is_Pow and expr.exp.is_Rational and abs(expr.exp) is S.Half


def sqrt_depth(p):
    """Return the maximum depth of any square root argument of p.

    >>> from sympy.functions.elementary.miscellaneous import sqrt
    >>> from sympy.simplify.sqrtdenest import sqrt_depth

    Neither of these square roots contains any other square roots
    so the depth is 1:

    >>> sqrt_depth(1 + sqrt(2)*(1 + sqrt(3)))
    1

    The sqrt(3) is contained within a square root so the depth is
    2:

    >>> sqrt_depth(1 + sqrt(2)*sqrt(1 + sqrt(3)))
    2
    """

    if p.is_Atom:
        return 0
    elif p.is_Add or p.is_Mul:
        return max([sqrt_depth(x) for x in p.args])
    elif is_sqrt(p):
        return sqrt_depth(p.base) + 1
    else:
        return 0


def is_algebraic(p):
    """Return True if p is comprised of only Rationals or square roots
    of Rationals and algebraic operations.

    Examples
    ========
    >>> from sympy.functions.elementary.miscellaneous import sqrt
    >>> from sympy.simplify.sqrtdenest import is_algebraic
    >>> from sympy import cos
    >>> is_algebraic(sqrt(2)*(3/(sqrt(7) + sqrt(5)*sqrt(2))))
    True
    >>> is_algebraic(sqrt(2)*(3/(sqrt(7) + sqrt(5)*cos(2))))
    False
    """

    if p.is_Rational:
        return True
    elif p.is_Atom:
        return False
    elif is_sqrt(p) or p.is_Pow and p.exp.is_Integer:
        return is_algebraic(p.base)
    elif p.is_Add or p.is_Mul:
        return all(is_algebraic(x) for x in p.args)
    else:
        return False


def _subsets(n):
    """
    Returns all possible subsets of the set (0, 1, ..., n-1) except the
    empty set, listed in reversed lexicographical order according to binary
    representation, so that the case of the fourth root is treated last.

    Examples
    ========

    >>> from sympy.simplify.sqrtdenest import _subsets
    >>> _subsets(2)
    [[1, 0], [0, 1], [1, 1]]

    """
    if n == 1:
        a = [[1]]
    elif n == 2:
        a = [[1, 0], [0, 1], [1, 1]]
    elif n == 3:
        a = [[1, 0, 0], [0, 1, 0], [1, 1, 0],
             [0, 0, 1], [1, 0, 1], [0, 1, 1], [1, 1, 1]]
    else:
        b = _subsets(n - 1)
        a0 = [x + [0] for x in b]
        a1 = [x + [1] for x in b]
        a = a0 + [[0]*(n - 1) + [1]] + a1
    return a


[docs]def sqrtdenest(expr, max_iter=3): """Denests sqrts in an expression that contain other square roots if possible, otherwise returns the expr unchanged. This is based on the algorithms of [1]. Examples ======== >>> from sympy.simplify.sqrtdenest import sqrtdenest >>> from sympy import sqrt >>> sqrtdenest(sqrt(5 + 2 * sqrt(6))) sqrt(2) + sqrt(3) See Also ======== sympy.solvers.solvers.unrad References ========== [1] http://www.almaden.ibm.com/cs/people/fagin/symb85.pdf [2] D. J. Jeffrey and A. D. Rich, 'Symplifying Square Roots of Square Roots by Denesting' (available at http://www.cybertester.com/data/denest.pdf) """ expr = expand_mul(sympify(expr)) for i in range(max_iter): z = _sqrtdenest0(expr) if expr == z: return expr expr = z return expr
def _sqrt_match(p): """Return [a, b, r] for p.match(a + b*sqrt(r)) where, in addition to matching, sqrt(r) also has then maximal sqrt_depth among addends of p. Examples ======== >>> from sympy.functions.elementary.miscellaneous import sqrt >>> from sympy.simplify.sqrtdenest import _sqrt_match >>> _sqrt_match(1 + sqrt(2) + sqrt(2)*sqrt(3) + 2*sqrt(1+sqrt(5))) [1 + sqrt(2) + sqrt(6), 2, 1 + sqrt(5)] """ from sympy.simplify.simplify import split_surds p = _mexpand(p) if p.is_Number: res = (p, S.Zero, S.Zero) elif p.is_Add: pargs = sorted(p.args, key=default_sort_key) if all((x**2).is_Rational for x in pargs): r, b, a = split_surds(p) res = a, b, r return list(res) # to make the process canonical, the argument is included in the tuple # so when the max is selected, it will be the largest arg having a # given depth v = [(sqrt_depth(x), x, i) for i, x in enumerate(pargs)] nmax = max(v) if nmax[0] == 0: res = [] else: # select r depth, _, i = nmax r = pargs.pop(i) v.pop(i) b = S.One if r.is_Mul: bv = [] rv = [] for x in r.args: if sqrt_depth(x) < depth: bv.append(x) else: rv.append(x) b = Mul._from_args(bv) r = Mul._from_args(rv) # collect terms comtaining r a1 = [] b1 = [b] for x in v: if x[0] < depth: a1.append(x[1]) else: x1 = x[1] if x1 == r: b1.append(1) else: if x1.is_Mul: x1args = list(x1.args) if r in x1args: x1args.remove(r) b1.append(Mul(*x1args)) else: a1.append(x[1]) else: a1.append(x[1]) a = Add(*a1) b = Add(*b1) #a = Add._from_args(pargs) res = (a, b, r**2) else: b, r = p.as_coeff_Mul() if is_sqrt(r): res = (S.Zero, b, r**2) else: res = [] return list(res) class SqrtdenestStopIteration(StopIteration): pass def _sqrtdenest0(expr): """Returns expr after denesting its arguments.""" if is_sqrt(expr): n, d = expr.as_numer_denom() if d is S.One: # n is a square root if n.base.is_Add: args = sorted(n.base.args, key=default_sort_key) if len(args) > 2 and all((x**2).is_Integer for x in args): try: return _sqrtdenest_rec(n) except SqrtdenestStopIteration: pass expr = sqrt(_mexpand(Add(*[_sqrtdenest0(x) for x in args]))) return _sqrtdenest1(expr) else: n, d = [_sqrtdenest0(i) for i in (n, d)] return n/d if isinstance(expr, Expr): args = expr.args if args: return expr.func(*[_sqrtdenest0(a) for a in args]) return expr def _sqrtdenest_rec(expr): """Helper that denests the square root of three or more surds. It returns the denested expression; if it cannot be denested it throws SqrtdenestStopIteration Algorithm: expr.base is in the extension Q_m = Q(sqrt(r_1),..,sqrt(r_k)); split expr.base = a + b*sqrt(r_k), where `a` and `b` are on Q_(m-1) = Q(sqrt(r_1),..,sqrt(r_(k-1))); then a**2 - b**2*r_k is on Q_(m-1); denest sqrt(a**2 - b**2*r_k) and so on. See [1], section 6. Examples ======== >>> from sympy import sqrt >>> from sympy.simplify.sqrtdenest import _sqrtdenest_rec >>> _sqrtdenest_rec(sqrt(-72*sqrt(2) + 158*sqrt(5) + 498)) -sqrt(10) + sqrt(2) + 9 + 9*sqrt(5) >>> w=-6*sqrt(55)-6*sqrt(35)-2*sqrt(22)-2*sqrt(14)+2*sqrt(77)+6*sqrt(10)+65 >>> _sqrtdenest_rec(sqrt(w)) -sqrt(11) - sqrt(7) + sqrt(2) + 3*sqrt(5) """ from sympy.simplify.simplify import radsimp, split_surds, rad_rationalize if not expr.is_Pow: return sqrtdenest(expr) if expr.base < 0: return sqrt(-1)*_sqrtdenest_rec(sqrt(-expr.base)) g, a, b = split_surds(expr.base) a = a*sqrt(g) if a < b: a, b = b, a c2 = _mexpand(a**2 - b**2) if len(c2.args) > 2: g, a1, b1 = split_surds(c2) a1 = a1*sqrt(g) if a1 < b1: a1, b1 = b1, a1 c2_1 = _mexpand(a1**2 - b1**2) c_1 = _sqrtdenest_rec(sqrt(c2_1)) d_1 = _sqrtdenest_rec(sqrt(a1 + c_1)) num, den = rad_rationalize(b1, d_1) c = _mexpand(d_1/sqrt(2) + num/(den*sqrt(2))) else: c = _sqrtdenest1(sqrt(c2)) if sqrt_depth(c) > 1: raise SqrtdenestStopIteration ac = a + c if len(ac.args) >= len(expr.args): if count_ops(ac) >= count_ops(expr.base): raise SqrtdenestStopIteration d = sqrtdenest(sqrt(ac)) if sqrt_depth(d) > 1: raise SqrtdenestStopIteration num, den = rad_rationalize(b, d) r = d/sqrt(2) + num/(den*sqrt(2)) r = radsimp(r) return _mexpand(r) def _sqrtdenest1(expr, denester=True): """Return denested expr after denesting with simpler methods or, that failing, using the denester.""" from sympy.simplify.simplify import radsimp if not is_sqrt(expr): return expr a = expr.base if a.is_Atom: return expr val = _sqrt_match(a) if not val: return expr a, b, r = val # try a quick numeric denesting d2 = _mexpand(a**2 - b**2*r) if d2.is_Rational: if d2.is_positive: z = _sqrt_numeric_denest(a, b, r, d2) if z is not None: return z else: # fourth root case # sqrtdenest(sqrt(3 + 2*sqrt(3))) = # sqrt(2)*3**(1/4)/2 + sqrt(2)*3**(3/4)/2 dr2 = _mexpand(-d2*r) dr = sqrt(dr2) if dr.is_Rational: z = _sqrt_numeric_denest(_mexpand(b*r), a, r, dr2) if z is not None: return z/root(r, 4) else: z = _sqrt_symbolic_denest(a, b, r) if z is not None: return z if not denester or not is_algebraic(expr): return expr res = sqrt_biquadratic_denest(expr, a, b, r, d2) if res: return res # now call to the denester av0 = [a, b, r, d2] z = _denester([radsimp(expr**2)], av0, 0, sqrt_depth(expr))[0] if av0[1] is None: return expr if z is not None: if sqrt_depth(z) == sqrt_depth(expr) and count_ops(z) > count_ops(expr): return expr return z return expr def _sqrt_symbolic_denest(a, b, r): """Given an expression, sqrt(a + b*sqrt(b)), return the denested expression or None. Algorithm: If r = ra + rb*sqrt(rr), try replacing sqrt(rr) in ``a`` with (y**2 - ra)/rb, and if the result is a quadratic, ca*y**2 + cb*y + cc, and (cb + b)**2 - 4*ca*cc is 0, then sqrt(a + b*sqrt(r)) can be rewritten as sqrt(ca*(sqrt(r) + (cb + b)/(2*ca))**2). Examples ======== >>> from sympy.simplify.sqrtdenest import _sqrt_symbolic_denest, sqrtdenest >>> from sympy import sqrt, Symbol >>> from sympy.abc import x >>> a, b, r = 16 - 2*sqrt(29), 2, -10*sqrt(29) + 55 >>> _sqrt_symbolic_denest(a, b, r) sqrt(-2*sqrt(29) + 11) + sqrt(5) If the expression is numeric, it will be simplified: >>> w = sqrt(sqrt(sqrt(3) + 1) + 1) + 1 + sqrt(2) >>> sqrtdenest(sqrt((w**2).expand())) 1 + sqrt(2) + sqrt(1 + sqrt(1 + sqrt(3))) Otherwise, it will only be simplified if assumptions allow: >>> w = w.subs(sqrt(3), sqrt(x + 3)) >>> sqrtdenest(sqrt((w**2).expand())) sqrt((sqrt(sqrt(sqrt(x + 3) + 1) + 1) + 1 + sqrt(2))**2) Notice that the argument of the sqrt is a square. If x is made positive then the sqrt of the square is resolved: >>> _.subs(x, Symbol('x', positive=True)) sqrt(sqrt(sqrt(x + 3) + 1) + 1) + 1 + sqrt(2) """ a, b, r = map(sympify, (a, b, r)) rval = _sqrt_match(r) if not rval: return None ra, rb, rr = rval if rb: y = Dummy('y', positive=True) try: newa = Poly(a.subs(sqrt(rr), (y**2 - ra)/rb), y) except PolynomialError: return None if newa.degree() == 2: ca, cb, cc = newa.all_coeffs() cb += b if _mexpand(cb**2 - 4*ca*cc).equals(0): z = sqrt(ca*(sqrt(r) + cb/(2*ca))**2) if z.is_number: z = _mexpand(Mul._from_args(z.as_content_primitive())) return z def _sqrt_numeric_denest(a, b, r, d2): """Helper that denest expr = a + b*sqrt(r), with d2 = a**2 - b**2*r > 0 or returns None if not denested. """ from sympy.simplify.simplify import radsimp depthr = sqrt_depth(r) d = sqrt(d2) vad = a + d # sqrt_depth(res) <= sqrt_depth(vad) + 1 # sqrt_depth(expr) = depthr + 2 # there is denesting if sqrt_depth(vad)+1 < depthr + 2 # if vad**2 is Number there is a fourth root if sqrt_depth(vad) < depthr + 1 or (vad**2).is_Rational: vad1 = radsimp(1/vad) return (sqrt(vad/2) + sign(b)*sqrt((b**2*r*vad1/2).expand())).expand() def sqrt_biquadratic_denest(expr, a, b, r, d2): """denest expr = sqrt(a + b*sqrt(r)) where a, b, r are linear combinations of square roots of positive rationals on the rationals (SQRR) and r > 0, b != 0, d2 = a**2 - b**2*r > 0 If it cannot denest it returns None. ALGORITHM Search for a solution A of type SQRR of the biquadratic equation 4*A**4 - 4*a*A**2 + b**2*r = 0 (1) sqd = sqrt(a**2 - b**2*r) Choosing the sqrt to be positive, the possible solutions are A = sqrt(a/2 +/- sqd/2) Since a, b, r are SQRR, then a**2 - b**2*r is a SQRR, so if sqd can be denested, it is done by _sqrtdenest_rec, and the result is a SQRR. Similarly for A. Examples of solutions (in both cases a and sqd are positive): Example of expr with solution sqrt(a/2 + sqd/2) but not solution sqrt(a/2 - sqd/2): expr = sqrt(-sqrt(15) - sqrt(2)*sqrt(-sqrt(5) + 5) - sqrt(3) + 8) a = -sqrt(15) - sqrt(3) + 8; sqd = -2*sqrt(5) - 2 + 4*sqrt(3) Example of expr with solution sqrt(a/2 - sqd/2) but not solution sqrt(a/2 + sqd/2): w = 2 + r2 + r3 + (1 + r3)*sqrt(2 + r2 + 5*r3) expr = sqrt((w**2).expand()) a = 4*sqrt(6) + 8*sqrt(2) + 47 + 28*sqrt(3) sqd = 29 + 20*sqrt(3) Define B = b/2*A; eq.(1) implies a = A**2 + B**2*r; then expr**2 = a + b*sqrt(r) = (A + B*sqrt(r))**2 Examples ======== >>> from sympy import sqrt >>> from sympy.simplify.sqrtdenest import _sqrt_match, sqrt_biquadratic_denest >>> z = sqrt((2*sqrt(2) + 4)*sqrt(2 + sqrt(2)) + 5*sqrt(2) + 8) >>> a, b, r = _sqrt_match(z**2) >>> d2 = a**2 - b**2*r >>> sqrt_biquadratic_denest(z, a, b, r, d2) sqrt(2) + sqrt(sqrt(2) + 2) + 2 """ from sympy.simplify.simplify import radsimp, rad_rationalize if r <= 0 or d2 < 0 or not b or sqrt_depth(expr.base) < 2: return None for x in (a, b, r): for y in x.args: y2 = y**2 if not y2.is_Integer or not y2.is_positive: return None sqd = _mexpand(sqrtdenest(sqrt(radsimp(d2)))) if sqrt_depth(sqd) > 1: return None x1, x2 = [a/2 + sqd/2, a/2 - sqd/2] # look for a solution A with depth 1 for x in (x1, x2): A = sqrtdenest(sqrt(x)) if sqrt_depth(A) > 1: continue Bn, Bd = rad_rationalize(b, _mexpand(2*A)) B = Bn/Bd z = A + B*sqrt(r) if z < 0: z = -z return _mexpand(z) return None def _denester(nested, av0, h, max_depth_level): """Denests a list of expressions that contain nested square roots. Algorithm based on <http://www.almaden.ibm.com/cs/people/fagin/symb85.pdf>. It is assumed that all of the elements of 'nested' share the same bottom-level radicand. (This is stated in the paper, on page 177, in the paragraph immediately preceding the algorithm.) When evaluating all of the arguments in parallel, the bottom-level radicand only needs to be denested once. This means that calling _denester with x arguments results in a recursive invocation with x+1 arguments; hence _denester has polynomial complexity. However, if the arguments were evaluated separately, each call would result in two recursive invocations, and the algorithm would have exponential complexity. This is discussed in the paper in the middle paragraph of page 179. """ from sympy.simplify.simplify import radsimp if h > max_depth_level: return None, None if av0[1] is None: return None, None if (av0[0] is None and all(n.is_Number for n in nested)): # no arguments are nested for f in _subsets(len(nested)): # test subset 'f' of nested p = _mexpand(Mul(*[nested[i] for i in range(len(f)) if f[i]])) if f.count(1) > 1 and f[-1]: p = -p sqp = sqrt(p) if sqp.is_Rational: return sqp, f # got a perfect square so return its square root. # Otherwise, return the radicand from the previous invocation. return sqrt(nested[-1]), [0]*len(nested) else: R = None if av0[0] is not None: values = [av0[:2]] R = av0[2] nested2 = [av0[3], R] av0[0] = None else: values = filter(None, [_sqrt_match(expr) for expr in nested]) for v in values: if v[2]: # Since if b=0, r is not defined if R is not None: if R != v[2]: av0[1] = None return None, None else: R = v[2] if R is None: # return the radicand from the previous invocation return sqrt(nested[-1]), [0]*len(nested) nested2 = [_mexpand(v[0]**2) - _mexpand(R*v[1]**2) for v in values] + [R] d, f = _denester(nested2, av0, h + 1, max_depth_level) #d = sqrtdenest(d) if not f: return None, None if not any(f[i] for i in range(len(nested))): v = values[-1] return sqrt(v[0] + _mexpand(v[1]*d)), f else: p = Mul(*[nested[i] for i in range(len(nested)) if f[i]]) v = _sqrt_match(p) if 1 in f and f.index(1) < len(nested) - 1 and f[len(nested) - 1]: v[0] = -v[0] v[1] = -v[1] if not f[len(nested)]: # Solution denests with square roots vad = _mexpand(v[0] + d) if vad <= 0: # return the radicand from the previous invocation. return sqrt(nested[-1]), [0]*len(nested) if not(sqrt_depth(vad) <= sqrt_depth(R) + 1 or (vad**2).is_Number): av0[1] = None return None, None sqvad = _sqrtdenest1(sqrt(vad), denester=False) if not (sqrt_depth(sqvad) <= sqrt_depth(R) + 1): av0[1] = None return None, None sqvad1 = radsimp(1/sqvad) res = _mexpand(sqvad/sqrt(2) + (v[1]*sqrt(R)*sqvad1/sqrt(2))) return res, f # sign(v[1])*sqrt(_mexpand(v[1]**2*R*vad1/2))), f else: # Solution requires a fourth root s2 = _mexpand(v[1]*R) + d if s2 <= 0: return sqrt(nested[-1]), [0]*len(nested) FR, s = root(_mexpand(R), 4), sqrt(s2) return _mexpand(s/(sqrt(2)*FR) + v[0]*FR/(sqrt(2)*s)), f