.. |newpage| raw:: latex \newpage .. |cr| raw:: latex \hspace{0.0mm} |newpage| Mpmath: Number identification =============================================================================== Most function in mpmath are concerned with producing approximations from exact mathematical formulas. It is also useful to consider the inverse problem: given only a decimal approximation for a number, such as 0.7320508075688772935274463, is it possible to find an exact formula? Subject to certain restrictions, such "reverse engineering" is indeed possible thanks to the existence of *integer relation algorithms*. Mpmath implements the PSLQ algorithm (developed by H. Ferguson), which is one such algorithm. Automated number recognition based on PSLQ is not a silver bullet. Any occurring transcendental constants (`\pi`, `e`, etc) must be guessed by the user, and the relation between those constants in the formula must be linear (such as `x = 3 \pi + 4 e`). More complex formulas can be found by combining PSLQ with functional transformations; however, this is only feasible to a limited extent since the computation time grows exponentially with the number of operations that need to be combined. This is a fairly direct translation to Python of the pseudocode described in :cite:t:`Ferguson1991` and :cite:t:`Ferguson1995`. .. _rst_mpm_pslq: Constant recognition ----------------------------------- .. method:: ctx.pslq(x, tol=None, maxcoeff=1000, maxsteps=100, verbose=False) where ``ctx`` is ``dec``, ``mpm``, or ``gmp``. Given a vector of real numbers `x = [x_0, x_1, ..., x_n]`, ``pslq(x)`` uses the PSLQ algorithm to find a list of integers `[c_0, c_1, ..., c_n]` such that .. math :: |c_1 x_1 + c_2 x_2 + ... + c_n x_n| < \mathrm{tol} and such that `\max |c_k| < \mathrm{maxcoeff}`. If no such vector exists, :func:`~mpmath.pslq` returns ``None``. The tolerance defaults to 3/4 of the working precision. **Examples** Find rational approximations for `\pi`: .. code-block:: pycon >>> from mpmath import * >>> mp.dps = 15; mp.pretty = True >>> pslq([-1, pi], tol=0.01) [22, 7] >>> pslq([-1, pi], tol=0.001) [355, 113] >>> mpf(22)/7; mpf(355)/113; +pi 3.14285714285714 3.14159292035398 3.14159265358979 Pi is not a rational number with denominator less than 1000: .. code-block:: pycon >>> pslq([-1, pi]) >>> To within the standard precision, it can however be approximated by at least one rational number with denominator less than `10^{12}`: .. code-block:: pycon >>> p, q = pslq([-1, pi], maxcoeff=10**12) >>> print(p); print(q) 238410049439 75888275702 >>> mpf(p)/q 3.14159265358979 The PSLQ algorithm can be applied to long vectors. For example, we can investigate the rational (in)dependence of integer square roots: .. code-block:: pycon >>> mp.dps = 30 >>> pslq([sqrt(n) for n in range(2, 5+1)]) >>> >>> pslq([sqrt(n) for n in range(2, 6+1)]) >>> >>> pslq([sqrt(n) for n in range(2, 8+1)]) [2, 0, 0, 0, 0, 0, -1] **Machin formulas** A famous formula for `\pi` is Machin's, .. math :: \frac{\pi}{4} = 4 \operatorname{acot} 5 - \operatorname{acot} 239 There are actually infinitely many formulas of this type. Two others are .. math :: \frac{\pi}{4} = \operatorname{acot} 1 \frac{\pi}{4} = 12 \operatorname{acot} 49 + 32 \operatorname{acot} 57 + 5 \operatorname{acot} 239 + 12 \operatorname{acot} 110443 We can easily verify the formulas using the PSLQ algorithm: .. code-block:: pycon >>> mp.dps = 30 >>> pslq([pi/4, acot(1)]) [1, -1] >>> pslq([pi/4, acot(5), acot(239)]) [1, -4, 1] >>> pslq([pi/4, acot(49), acot(57), acot(239), acot(110443)]) [1, -12, -32, 5, -12] We could try to generate a custom Machin-like formula by running the PSLQ algorithm with a few inverse cotangent values, for example acot(2), acot(3) ... acot(10). Unfortunately, there is a linear dependence among these values, resulting in only that dependence being detected, with a zero coefficient for `\pi`: .. code-block:: pycon >>> pslq([pi] + [acot(n) for n in range(2,11)]) [0, 1, -1, 0, 0, 0, -1, 0, 0, 0] We get better luck by removing linearly dependent terms: .. code-block:: pycon >>> pslq([pi] + [acot(n) for n in range(2,11) if n not in (3, 5)]) [1, -8, 0, 0, 4, 0, 0, 0] In other words, we found the following formula: .. code-block:: pycon >>> 8*acot(2) - 4*acot(7) 3.14159265358979323846264338328 >>> +pi 3.14159265358979323846264338328 .. _rst_mpm_findpoly: Algebraic identification --------------------------------------- .. method:: ctx.findpoly(x, n=1, **kwargs) where ``ctx`` is ``dec``, ``mpm``, or ``gmp``. ``findpoly(x, n)`` returns the coefficients of an integer polynomial `P` of degree at most `n` such that `P(x) \approx 0`. If no polynomial having `x` as a root can be found, :func:`~mpmath.findpoly` returns ``None``. :func:`~mpmath.findpoly` works by successively calling :func:`~mpmath.pslq` with the vectors `[1, x]`, `[1, x, x^2]`, `[1, x, x^2, x^3]`, ..., `[1, x, x^2, .., x^n]` as input. Keyword arguments given to :func:`~mpmath.findpoly` are forwarded verbatim to :func:`~mpmath.pslq`. In particular, you can specify a tolerance for `P(x)` with ``tol`` and a maximum permitted coefficient size with ``maxcoeff``. For large values of `n`, it is recommended to run :func:`~mpmath.findpoly` at high precision; preferably 50 digits or more. **Examples** By default (degree `n = 1`), :func:`~mpmath.findpoly` simply finds a linear polynomial with a rational root: .. code-block:: pycon >>> from mpmath import * >>> mp.dps = 15; mp.pretty = True >>> findpoly(0.7) [-10, 7] The generated coefficient list is valid input to ``polyval`` and ``polyroots``: .. code-block:: pycon >>> nprint(polyval(findpoly(phi, 2), phi), 1) -2.0e-16 >>> for r in polyroots(findpoly(phi, 2)): ... print(r) ... -0.618033988749895 1.61803398874989 Numbers of the form `m + n \sqrt p` for integers `(m, n, p)` are solutions to quadratic equations. As we find here, `1+\sqrt 2` is a root of the polynomial `x^2 - 2x - 1`: .. code-block:: pycon >>> findpoly(1+sqrt(2), 2) [1, -2, -1] >>> findroot(lambda x: x**2 - 2*x - 1, 1) 2.4142135623731 Despite only containing square roots, the following number results in a polynomial of degree 4: .. code-block:: pycon >>> findpoly(sqrt(2)+sqrt(3), 4) [1, 0, -10, 0, 1] In fact, `x^4 - 10x^2 + 1` is the *minimal polynomial* of `r = \sqrt 2 + \sqrt 3`, meaning that a rational polynomial of lower degree having `r` as a root does not exist. Given sufficient precision, :func:`~mpmath.findpoly` will usually find the correct minimal polynomial of a given algebraic number. **Non-algebraic numbers** If :func:`~mpmath.findpoly` fails to find a polynomial with given coefficient size and tolerance constraints, that means no such polynomial exists. We can verify that `\pi` is not an algebraic number of degree 3 with coefficients less than 1000: .. code-block:: pycon >>> mp.dps = 15 >>> findpoly(pi, 3) >>> It is always possible to find an algebraic approximation of a number using one (or several) of the following methods: 1. Increasing the permitted degree 2. Allowing larger coefficients 3. Reducing the tolerance One example of each method is shown below: .. code-block:: pycon >>> mp.dps = 15 >>> findpoly(pi, 4) [95, -545, 863, -183, -298] >>> findpoly(pi, 3, maxcoeff=10000) [836, -1734, -2658, -457] >>> findpoly(pi, 3, tol=1e-7) [-4, 22, -29, -2] It is unknown whether Euler's constant is transcendental (or even irrational). We can use :func:`~mpmath.findpoly` to check that if is an algebraic number, its minimal polynomial must have degree at least 7 and a coefficient of magnitude at least 1000000: .. code-block:: pycon >>> mp.dps = 200 >>> findpoly(euler, 6, maxcoeff=10**6, tol=1e-100, maxsteps=1000) >>> Note that the high precision and strict tolerance is necessary for such high-degree runs, since otherwise unwanted low-accuracy approximations will be detected. It may also be necessary to set maxsteps high to prevent a premature exit (before the coefficient bound has been reached). Running with ``verbose=True`` to get an idea what is happening can be useful. .. _rst_mpm_identify: Integer relations (PSLQ) ---------------------------- .. method:: ctx.identify(x, constants=[], tol=None, maxcoeff=1000, full=False, verbose=False) where ``ctx`` is ``dec``, ``mpm``, or ``gmp``. Given a real number `x`, ``identify(x)`` attempts to find an exact formula for `x`. This formula is returned as a string. If no match is found, ``None`` is returned. With ``full=True``, a list of matching formulas is returned. As a simple example, :func:`~mpmath.identify` will find an algebraic formula for the golden ratio: .. code-block:: pycon >>> from mpmath import * >>> mp.dps = 15; mp.pretty = True >>> identify(phi) '((1+sqrt(5))/2)' :func:`~mpmath.identify` can identify simple algebraic numbers and simple combinations of given base constants, as well as certain basic transformations thereof. More specifically, :func:`~mpmath.identify` looks for the following: 1. Fractions 2. Quadratic algebraic numbers 3. Rational linear combinations of the base constants 4. Any of the above after first transforming `x` into `f(x)` where `f(x)` is `1/x`, `\sqrt x`, `x^2`, `\log x` or `\exp x`, either directly or with `x` or `f(x)` multiplied or divided by one of the base constants 5. Products of fractional powers of the base constants and small integers Base constants can be given as a list of strings representing mpmath expressions (:func:`~mpmath.identify` will ``eval`` the strings to numerical values and use the original strings for the output), or as a dict of formula:value pairs. In order not to produce spurious results, :func:`~mpmath.identify` should be used with high precision; preferably 50 digits or more. **Examples** Simple identifications can be performed safely at standard precision. Here the default recognition of rational, algebraic, and exp/log of algebraic numbers is demonstrated: .. code-block:: pycon >>> mp.dps = 15 >>> identify(0.22222222222222222) '(2/9)' >>> identify(1.9662210973805663) 'sqrt(((24+sqrt(48))/8))' >>> identify(4.1132503787829275) 'exp((sqrt(8)/2))' >>> identify(0.881373587019543) 'log(((2+sqrt(8))/2))' By default, :func:`~mpmath.identify` does not recognize `\pi`. At standard precision it finds a not too useful approximation. At slightly increased precision, this approximation is no longer accurate enough and :func:`~mpmath.identify` more correctly returns ``None``: .. code-block:: pycon >>> identify(pi) '(2**(176/117)*3**(20/117)*5**(35/39))/(7**(92/117))' >>> mp.dps = 30 >>> identify(pi) >>> Numbers such as `\pi`, and simple combinations of user-defined constants, can be identified if they are provided explicitly: .. code-block:: pycon >>> identify(3*pi-2*e, ['pi', 'e']) '(3*pi + (-2)*e)' Here is an example using a dict of constants. Note that the constants need not be "atomic"; :func:`~mpmath.identify` can just as well express the given number in terms of expressions given by formulas: .. code-block:: pycon >>> identify(pi+e, {'a':pi+2, 'b':2*e}) '((-2) + 1*a + (1/2)*b)' Next, we attempt some identifications with a set of base constants. It is necessary to increase the precision a bit. .. code-block:: pycon >>> mp.dps = 50 >>> base = ['sqrt(2)','pi','log(2)'] >>> identify(0.25, base) '(1/4)' >>> identify(3*pi + 2*sqrt(2) + 5*log(2)/7, base) '(2*sqrt(2) + 3*pi + (5/7)*log(2))' >>> identify(exp(pi+2), base) 'exp((2 + 1*pi))' >>> identify(1/(3+sqrt(2)), base) '((3/7) + (-1/7)*sqrt(2))' >>> identify(sqrt(2)/(3*pi+4), base) 'sqrt(2)/(4 + 3*pi)' >>> identify(5**(mpf(1)/3)*pi*log(2)**2, base) '5**(1/3)*pi*log(2)**2' An example of an erroneous solution being found when too low precision is used: .. code-block:: pycon >>> mp.dps = 15 >>> identify(1/(3*pi-4*e+sqrt(8)), ['pi', 'e', 'sqrt(2)']) '((11/25) + (-158/75)*pi + (76/75)*e + (44/15)*sqrt(2))' >>> mp.dps = 50 >>> identify(1/(3*pi-4*e+sqrt(8)), ['pi', 'e', 'sqrt(2)']) '1/(3*pi + (-4)*e + 2*sqrt(2))' **Finding approximate solutions** The tolerance ``tol`` defaults to 3/4 of the working precision. Lowering the tolerance is useful for finding approximate matches. We can for example try to generate approximations for pi: .. code-block:: pycon >>> mp.dps = 15 >>> identify(pi, tol=1e-2) '(22/7)' >>> identify(pi, tol=1e-3) '(355/113)' >>> identify(pi, tol=1e-10) '(5**(339/269))/(2**(64/269)*3**(13/269)*7**(92/269))' With ``full=True``, and by supplying a few base constants, ``identify`` can generate almost endless lists of approximations for any number (the output below has been truncated to show only the first few): .. code-block:: pycon >>> for p in identify(pi, ['e', 'catalan'], tol=1e-5, full=True): ... print(p) ... # doctest: +ELLIPSIS e/log((6 + (-4/3)*e)) (3**3*5*e*catalan**2)/(2*7**2) sqrt(((-13) + 1*e + 22*catalan)) log(((-6) + 24*e + 4*catalan)/e) exp(catalan*((-1/5) + (8/15)*e)) catalan*(6 + (-6)*e + 15*catalan) sqrt((5 + 26*e + (-3)*catalan))/e e*sqrt(((-27) + 2*e + 25*catalan)) log(((-1) + (-11)*e + 59*catalan)) ((3/20) + (21/20)*e + (3/20)*catalan) ... The numerical values are roughly as close to `\pi` as permitted by the specified tolerance: .. code-block:: pycon >>> e/log(6-4*e/3) 3.14157719846001 >>> 135*e*catalan**2/98 3.14166950419369 >>> sqrt(e-13+22*catalan) 3.14158000062992 >>> log(24*e-6+4*catalan)-1 3.14158791577159 **Symbolic processing** The output formula can be evaluated as a Python expression. Note however that if fractions (like '2/3') are present in the formula, Python's :func:`~mpmath.eval()` may erroneously perform integer division. Note also that the output is not necessarily in the algebraically simplest form: .. code-block:: pycon >>> identify(sqrt(2)) '(sqrt(8)/2)' As a solution to both problems, consider using SymPy's :func:`~mpmath.sympify` to convert the formula into a symbolic expression. SymPy can be used to pretty-print or further simplify the formula symbolically: .. code-block:: pycon >>> from sympy import sympify # doctest: +SKIP >>> sympify(identify(sqrt(2))) # doctest: +SKIP 2**(1/2) Sometimes :func:`~mpmath.identify` can simplify an expression further than a symbolic algorithm: .. code-block:: pycon >>> from sympy import simplify # doctest: +SKIP >>> x = sympify('-1/(-3/2+(1/2)*5**(1/2))*(3/2-1/2*5**(1/2))**(1/2)') # doctest: +SKIP >>> x # doctest: +SKIP (3/2 - 5**(1/2)/2)**(-1/2) >>> x = simplify(x) # doctest: +SKIP >>> x # doctest: +SKIP 2/(6 - 2*5**(1/2))**(1/2) >>> mp.dps = 30 # doctest: +SKIP >>> x = sympify(identify(x.evalf(30))) # doctest: +SKIP >>> x # doctest: +SKIP 1/2 + 5**(1/2)/2 (In fact, this functionality is available directly in SymPy as the function :func:`~mpmath.nsimplify`, which is essentially a wrapper for :func:`~mpmath.identify`.) **Miscellaneous issues and limitations** The input `x` must be a real number. All base constants must be positive real numbers and must not be rationals or rational linear combinations of each other. The worst-case computation time grows quickly with the number of base constants. Already with 3 or 4 base constants, :func:`~mpmath.identify` may require several seconds to finish. To search for relations among a large number of constants, you should consider using :func:`~mpmath.pslq` directly. The extended transformations are applied to x, not the constants separately. As a result, ``identify`` will for example be able to recognize ``exp(2*pi+3)`` with ``pi`` given as a base constant, but not ``2*exp(pi)+3``. It will be able to recognize the latter if ``exp(pi)`` is given explicitly as a base constant.