Combinatorial¶
This module implements various combinatorial functions.
bell¶
-
class
sympy.functions.combinatorial.numbers.bell[source]¶ Bell numbers / Bell polynomials
The Bell numbers satisfy \(B_0 = 1\) and
\[B_n = \sum_{k=0}^{n-1} \binom{n-1}{k} B_k.\]They are also given by:
\[B_n = \frac{1}{e} \sum_{k=0}^{\infty} \frac{k^n}{k!}.\]The Bell polynomials are given by \(B_0(x) = 1\) and
\[B_n(x) = x \sum_{k=1}^{n-1} \binom{n-1}{k-1} B_{k-1}(x).\]The second kind of Bell polynomials (are sometimes called “partial” Bell polynomials or incomplete Bell polynomials) are defined as
\[B_{n,k}(x_1, x_2,\dotsc x_{n-k+1}) = \sum_{j_1+j_2+j_2+\dotsb=k \atop j_1+2j_2+3j_2+\dotsb=n} \frac{n!}{j_1!j_2!\dotsb j_{n-k+1}!} \left(\frac{x_1}{1!} \right)^{j_1} \left(\frac{x_2}{2!} \right)^{j_2} \dotsb \left(\frac{x_{n-k+1}}{(n-k+1)!} \right) ^{j_{n-k+1}}.\]- bell(n) gives the \(n^{th}\) Bell number, \(B_n\).
- bell(n, x) gives the \(n^{th}\) Bell polynomial, \(B_n(x)\).
- bell(n, k, (x1, x2, ...)) gives Bell polynomials of the second kind, \(B_{n,k}(x_1, x_2, \dotsc, x_{n-k+1})\).
See also
bernoulli,catalan,euler,fibonacci,harmonic,lucasNotes
Not to be confused with Bernoulli numbers and Bernoulli polynomials, which use the same notation.
References
[R88] http://en.wikipedia.org/wiki/Bell_number [R89] http://mathworld.wolfram.com/BellNumber.html [R90] http://mathworld.wolfram.com/BellPolynomial.html Examples
>>> from sympy import bell, Symbol, symbols
>>> [bell(n) for n in range(11)] [1, 1, 2, 5, 15, 52, 203, 877, 4140, 21147, 115975] >>> bell(30) 846749014511809332450147 >>> bell(4, Symbol('t')) t**4 + 6*t**3 + 7*t**2 + t >>> bell(6, 2, symbols('x:6')[1:]) 6*x1*x5 + 15*x2*x4 + 10*x3**2
bernoulli¶
-
class
sympy.functions.combinatorial.numbers.bernoulli[source]¶ Bernoulli numbers / Bernoulli polynomials
The Bernoulli numbers are a sequence of rational numbers defined by B_0 = 1 and the recursive relation (n > 0):
n ___ \ / n + 1 \ 0 = ) | | * B . /___ \ k / k k = 0
They are also commonly defined by their exponential generating function, which is x/(exp(x) - 1). For odd indices > 1, the Bernoulli numbers are zero.
The Bernoulli polynomials satisfy the analogous formula:
n ___ \ / n \ n-k B (x) = ) | | * B * x . n /___ \ k / k k = 0
Bernoulli numbers and Bernoulli polynomials are related as B_n(0) = B_n.
We compute Bernoulli numbers using Ramanujan’s formula:
/ n + 3 \ B = (A(n) - S(n)) / | | n \ n /
where A(n) = (n+3)/3 when n = 0 or 2 (mod 6), A(n) = -(n+3)/6 when n = 4 (mod 6), and:
[n/6] ___ \ / n + 3 \ S(n) = ) | | * B /___ \ n - 6*k / n-6*k k = 1
This formula is similar to the sum given in the definition, but cuts 2/3 of the terms. For Bernoulli polynomials, we use the formula in the definition.
- bernoulli(n) gives the nth Bernoulli number, B_n
- bernoulli(n, x) gives the nth Bernoulli polynomial in x, B_n(x)
See also
bell,catalan,euler,fibonacci,harmonic,lucasReferences
[R91] http://en.wikipedia.org/wiki/Bernoulli_number [R92] http://en.wikipedia.org/wiki/Bernoulli_polynomial [R93] http://mathworld.wolfram.com/BernoulliNumber.html [R94] http://mathworld.wolfram.com/BernoulliPolynomial.html Examples
>>> from sympy import bernoulli
>>> [bernoulli(n) for n in range(11)] [1, -1/2, 1/6, 0, -1/30, 0, 1/42, 0, -1/30, 0, 5/66] >>> bernoulli(1000001) 0
binomial¶
-
class
sympy.functions.combinatorial.factorials.binomial[source]¶ Implementation of the binomial coefficient. It can be defined in two ways depending on its desired interpretation:
C(n,k) = n!/(k!(n-k)!) or C(n, k) = ff(n, k)/k!First, in a strict combinatorial sense it defines the number of ways we can choose ‘k’ elements from a set of ‘n’ elements. In this case both arguments are nonnegative integers and binomial is computed using an efficient algorithm based on prime factorization.
The other definition is generalization for arbitrary ‘n’, however ‘k’ must also be nonnegative. This case is very useful when evaluating summations.
For the sake of convenience for negative ‘k’ this function will return zero no matter what valued is the other argument.
To expand the binomial when n is a symbol, use either expand_func() or expand(func=True). The former will keep the polynomial in factored form while the latter will expand the polynomial itself. See examples for details.
Examples
>>> from sympy import Symbol, Rational, binomial, expand_func >>> n = Symbol('n', integer=True, positive=True)
>>> binomial(15, 8) 6435
>>> binomial(n, -1) 0
Rows of Pascal’s triangle can be generated with the binomial function:
>>> for N in range(8): ... print([ binomial(N, i) for i in range(N + 1)]) ... [1] [1, 1] [1, 2, 1] [1, 3, 3, 1] [1, 4, 6, 4, 1] [1, 5, 10, 10, 5, 1] [1, 6, 15, 20, 15, 6, 1] [1, 7, 21, 35, 35, 21, 7, 1]
As can a given diagonal, e.g. the 4th diagonal:
>>> N = -4 >>> [ binomial(N, i) for i in range(1 - N)] [1, -4, 10, -20, 35]
>>> binomial(Rational(5, 4), 3) -5/128 >>> binomial(Rational(-5, 4), 3) -195/128
>>> binomial(n, 3) binomial(n, 3)
>>> binomial(n, 3).expand(func=True) n**3/6 - n**2/2 + n/3
>>> expand_func(binomial(n, 3)) n*(n - 2)*(n - 1)/6
catalan¶
-
class
sympy.functions.combinatorial.numbers.catalan[source]¶ Catalan numbers
The n-th catalan number is given by:
1 / 2*n \ C = ----- | | n n + 1 \ n /
- catalan(n) gives the n-th Catalan number, C_n
See also
bell,bernoulli,euler,fibonacci,harmonic,lucas,sympy.functions.combinatorial.factorials.binomialReferences
[R95] http://en.wikipedia.org/wiki/Catalan_number [R96] http://mathworld.wolfram.com/CatalanNumber.html [R97] http://functions.wolfram.com/GammaBetaErf/CatalanNumber/ [R98] http://geometer.org/mathcircles/catalan.pdf Examples
>>> from sympy import (Symbol, binomial, gamma, hyper, polygamma, ... catalan, diff, combsimp, Rational, I)
>>> [ catalan(i) for i in range(1,10) ] [1, 2, 5, 14, 42, 132, 429, 1430, 4862]
>>> n = Symbol("n", integer=True)
>>> catalan(n) catalan(n)
Catalan numbers can be transformed into several other, identical expressions involving other mathematical functions
>>> catalan(n).rewrite(binomial) binomial(2*n, n)/(n + 1)
>>> catalan(n).rewrite(gamma) 4**n*gamma(n + 1/2)/(sqrt(pi)*gamma(n + 2))
>>> catalan(n).rewrite(hyper) hyper((-n + 1, -n), (2,), 1)
For some non-integer values of n we can get closed form expressions by rewriting in terms of gamma functions:
>>> catalan(Rational(1,2)).rewrite(gamma) 8/(3*pi)
We can differentiate the Catalan numbers C(n) interpreted as a continuous real funtion in n:
>>> diff(catalan(n), n) (polygamma(0, n + 1/2) - polygamma(0, n + 2) + log(4))*catalan(n)
As a more advanced example consider the following ratio between consecutive numbers:
>>> combsimp((catalan(n + 1)/catalan(n)).rewrite(binomial)) 2*(2*n + 1)/(n + 2)
The Catalan numbers can be generalized to complex numbers:
>>> catalan(I).rewrite(gamma) 4**I*gamma(1/2 + I)/(sqrt(pi)*gamma(2 + I))
and evaluated with arbitrary precision:
>>> catalan(I).evalf(20) 0.39764993382373624267 - 0.020884341620842555705*I
euler¶
-
class
sympy.functions.combinatorial.numbers.euler[source]¶ Euler numbers
The euler numbers are given by:
2*n+1 k ___ ___ j 2*n+1 \ \ / k \ (-1) * (k-2*j) E = I ) ) | | -------------------- 2n /___ /___ \ j / k k k = 1 j = 0 2 * I * k E = 0 2n+1
- euler(n) gives the n-th Euler number, E_n
See also
bell,bernoulli,catalan,fibonacci,harmonic,lucasReferences
[R99] http://en.wikipedia.org/wiki/Euler_numbers [R100] http://mathworld.wolfram.com/EulerNumber.html [R101] http://en.wikipedia.org/wiki/Alternating_permutation [R102] http://mathworld.wolfram.com/AlternatingPermutation.html Examples
>>> from sympy import Symbol >>> from sympy.functions import euler >>> [euler(n) for n in range(10)] [1, 0, -1, 0, 5, 0, -61, 0, 1385, 0] >>> n = Symbol("n") >>> euler(n+2*n) euler(3*n)
factorial¶
-
class
sympy.functions.combinatorial.factorials.factorial[source]¶ Implementation of factorial function over nonnegative integers. By convention (consistent with the gamma function and the binomial coefficients), factorial of a negative integer is complex infinity.
The factorial is very important in combinatorics where it gives the number of ways in which \(n\) objects can be permuted. It also arises in calculus, probability, number theory, etc.
There is strict relation of factorial with gamma function. In fact n! = gamma(n+1) for nonnegative integers. Rewrite of this kind is very useful in case of combinatorial simplification.
Computation of the factorial is done using two algorithms. For small arguments naive product is evaluated. However for bigger input algorithm Prime-Swing is used. It is the fastest algorithm known and computes n! via prime factorization of special class of numbers, called here the ‘Swing Numbers’.
See also
factorial2,RisingFactorial,FallingFactorialExamples
>>> from sympy import Symbol, factorial, S >>> n = Symbol('n', integer=True)
>>> factorial(0) 1
>>> factorial(7) 5040
>>> factorial(-2) zoo
>>> factorial(n) factorial(n)
>>> factorial(2*n) factorial(2*n)
>>> factorial(S(1)/2) factorial(1/2)
subfactorial¶
-
class
sympy.functions.combinatorial.factorials.subfactorial[source]¶ The subfactorial counts the derangements of n items and is defined for non-negative integers as:
, | 1 for n = 0 !n = { 0 for n = 1 | (n - 1)*(!(n - 1) + !(n - 2)) for n > 1 `It can also be written as int(round(n!/exp(1))) but the recursive definition with caching is implemented for this function.
An interesting analytic expression is the following [R104]
\[!x = \Gamma(x + 1, -1)/e\]which is valid for non-negative integers x. The above formula is not very useful incase of non-integers. \(\Gamma(x + 1, -1)\) is single-valued only for integral arguments x, elsewhere on the positive real axis it has an infinite number of branches none of which are real.
See also
sympy.functions.combinatorial.factorials.factorial,sympy.utilities.iterables.generate_derangements,sympy.functions.special.gamma_functions.uppergammaReferences
[R103] http://en.wikipedia.org/wiki/Subfactorial [R104] (1, 2) http://mathworld.wolfram.com/Subfactorial.html Examples
>>> from sympy import subfactorial >>> from sympy.abc import n >>> subfactorial(n + 1) subfactorial(n + 1) >>> subfactorial(5) 44
factorial2 / double factorial¶
-
class
sympy.functions.combinatorial.factorials.factorial2[source]¶ The double factorial n!!, not to be confused with (n!)!
The double factorial is defined for nonnegative integers and for odd negative integers as:
, | n*(n - 2)*(n - 4)* ... * 1 for n positive odd n!! = { n*(n - 2)*(n - 4)* ... * 2 for n positive even | 1 for n = 0 | (n+2)!! / (n+2) for n negative odd `See also
factorial,RisingFactorial,FallingFactorialReferences
[R105] https://en.wikipedia.org/wiki/Double_factorial Examples
>>> from sympy import factorial2, var >>> var('n') n >>> factorial2(n + 1) factorial2(n + 1) >>> factorial2(5) 15 >>> factorial2(-1) 1 >>> factorial2(-5) 1/3
FallingFactorial¶
-
class
sympy.functions.combinatorial.factorials.FallingFactorial[source]¶ Falling factorial (related to rising factorial) is a double valued function arising in concrete mathematics, hypergeometric functions and series expansions. It is defined by
ff(x, k) = x * (x-1) * ... * (x - k+1)where ‘x’ can be arbitrary expression and ‘k’ is an integer. For more information check “Concrete mathematics” by Graham, pp. 66 or visit http://mathworld.wolfram.com/FallingFactorial.html page.
When x is a polynomial f of a single variable y of order >= 1, ff(x,k) = f(y) * f(y-1) * ... * f(x-k+1) as described in Peter Paule, “Greatest Factorial Factorization and Symbolic Summation”, Journal of Symbolic Computation, vol. 20, pp. 235-268, 1995.
>>> from sympy import ff, factorial, rf, gamma, polygamma, binomial, symbols >>> from sympy.abc import x, k >>> n, m = symbols('n m', integer=True) >>> ff(x, 0) 1 >>> ff(5, 5) 120 >>> ff(x, 5) == x*(x-1)*(x-2)*(x-3)*(x-4) True >>> ff(x**2, 2) Poly(x**4 - 2*x**3 + x**2, x, domain='ZZ') >>> ff(n, n) factorial(n)
Rewrite
>>> ff(x, k).rewrite(gamma) (-1)**k*gamma(k - x)/gamma(-x) >>> ff(x, k).rewrite(rf) RisingFactorial(-k + x + 1, k) >>> ff(x, m).rewrite(binomial) binomial(x, m)*factorial(m) >>> ff(n, m).rewrite(factorial) factorial(n)/factorial(-m + n)
See also
factorial,factorial2,RisingFactorialReferences
[R106] http://mathworld.wolfram.com/FallingFactorial.html
fibonacci¶
-
class
sympy.functions.combinatorial.numbers.fibonacci[source]¶ Fibonacci numbers / Fibonacci polynomials
The Fibonacci numbers are the integer sequence defined by the initial terms F_0 = 0, F_1 = 1 and the two-term recurrence relation F_n = F_{n-1} + F_{n-2}. This definition extended to arbitrary real and complex arguments using the formula
\[F_z = \frac{\phi^z - \cos(\pi z) \phi^{-z}}{\sqrt 5}\]The Fibonacci polynomials are defined by F_1(x) = 1, F_2(x) = x, and F_n(x) = x*F_{n-1}(x) + F_{n-2}(x) for n > 2. For all positive integers n, F_n(1) = F_n.
- fibonacci(n) gives the nth Fibonacci number, F_n
- fibonacci(n, x) gives the nth Fibonacci polynomial in x, F_n(x)
See also
bell,bernoulli,catalan,euler,harmonic,lucasReferences
[R107] http://en.wikipedia.org/wiki/Fibonacci_number [R108] http://mathworld.wolfram.com/FibonacciNumber.html Examples
>>> from sympy import fibonacci, Symbol
>>> [fibonacci(x) for x in range(11)] [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55] >>> fibonacci(5, Symbol('t')) t**4 + 3*t**2 + 1
harmonic¶
-
class
sympy.functions.combinatorial.numbers.harmonic[source]¶ Harmonic numbers
The nth harmonic number is given by \(\operatorname{H}_{n} = 1 + \frac{1}{2} + \frac{1}{3} + \ldots + \frac{1}{n}\).
More generally:
\[\operatorname{H}_{n,m} = \sum_{k=1}^{n} \frac{1}{k^m}\]As \(n \rightarrow \infty\), \(\operatorname{H}_{n,m} \rightarrow \zeta(m)\), the Riemann zeta function.
harmonic(n)gives the nth harmonic number, \(\operatorname{H}_n\)harmonic(n, m)gives the nth generalized harmonic number of order \(m\), \(\operatorname{H}_{n,m}\), whereharmonic(n) == harmonic(n, 1)
See also
bell,bernoulli,catalan,euler,fibonacci,lucasReferences
[R109] http://en.wikipedia.org/wiki/Harmonic_number [R110] http://functions.wolfram.com/GammaBetaErf/HarmonicNumber/ [R111] http://functions.wolfram.com/GammaBetaErf/HarmonicNumber2/ Examples
>>> from sympy import harmonic, oo
>>> [harmonic(n) for n in range(6)] [0, 1, 3/2, 11/6, 25/12, 137/60] >>> [harmonic(n, 2) for n in range(6)] [0, 1, 5/4, 49/36, 205/144, 5269/3600] >>> harmonic(oo, 2) pi**2/6
>>> from sympy import Symbol, Sum >>> n = Symbol("n")
>>> harmonic(n).rewrite(Sum) Sum(1/_k, (_k, 1, n))
We can evaluate harmonic numbers for all integral and positive rational arguments:
>>> from sympy import S, expand_func, simplify >>> harmonic(8) 761/280 >>> harmonic(11) 83711/27720
>>> H = harmonic(1/S(3)) >>> H harmonic(1/3) >>> He = expand_func(H) >>> He -log(6) - sqrt(3)*pi/6 + 2*Sum(log(sin(_k*pi/3))*cos(2*_k*pi/3), (_k, 1, 1)) + 3*Sum(1/(3*_k + 1), (_k, 0, 0)) >>> He.doit() -log(6) - sqrt(3)*pi/6 - log(sqrt(3)/2) + 3 >>> H = harmonic(25/S(7)) >>> He = simplify(expand_func(H).doit()) >>> He log(sin(pi/7)**(-2*cos(pi/7))*sin(2*pi/7)**(2*cos(16*pi/7))*cos(pi/14)**(-2*sin(pi/14))/14) + pi*tan(pi/14)/2 + 30247/9900 >>> He.n(40) 1.983697455232980674869851942390639915940 >>> harmonic(25/S(7)).n(40) 1.983697455232980674869851942390639915940
We can rewrite harmonic numbers in terms of polygamma functions:
>>> from sympy import digamma, polygamma >>> m = Symbol("m")
>>> harmonic(n).rewrite(digamma) polygamma(0, n + 1) + EulerGamma
>>> harmonic(n).rewrite(polygamma) polygamma(0, n + 1) + EulerGamma
>>> harmonic(n,3).rewrite(polygamma) polygamma(2, n + 1)/2 - polygamma(2, 1)/2
>>> harmonic(n,m).rewrite(polygamma) (-1)**m*(polygamma(m - 1, 1) - polygamma(m - 1, n + 1))/factorial(m - 1)
Integer offsets in the argument can be pulled out:
>>> from sympy import expand_func
>>> expand_func(harmonic(n+4)) harmonic(n) + 1/(n + 4) + 1/(n + 3) + 1/(n + 2) + 1/(n + 1)
>>> expand_func(harmonic(n-4)) harmonic(n) - 1/(n - 1) - 1/(n - 2) - 1/(n - 3) - 1/n
Some limits can be computed as well:
>>> from sympy import limit, oo
>>> limit(harmonic(n), n, oo) oo
>>> limit(harmonic(n, 2), n, oo) pi**2/6
>>> limit(harmonic(n, 3), n, oo) -polygamma(2, 1)/2
However we can not compute the general relation yet:
>>> limit(harmonic(n, m), n, oo) harmonic(oo, m)
which equals
zeta(m)form > 1.
lucas¶
-
class
sympy.functions.combinatorial.numbers.lucas[source]¶ Lucas numbers
Lucas numbers satisfy a recurrence relation similar to that of the Fibonacci sequence, in which each term is the sum of the preceding two. They are generated by choosing the initial values L_0 = 2 and L_1 = 1.
- lucas(n) gives the nth Lucas number
See also
bell,bernoulli,catalan,euler,fibonacci,harmonicReferences
[R112] http://en.wikipedia.org/wiki/Lucas_number [R113] http://mathworld.wolfram.com/LucasNumber.html Examples
>>> from sympy import lucas
>>> [lucas(x) for x in range(11)] [2, 1, 3, 4, 7, 11, 18, 29, 47, 76, 123]
RisingFactorial¶
-
class
sympy.functions.combinatorial.factorials.RisingFactorial[source]¶ Rising factorial (also called Pochhammer symbol) is a double valued function arising in concrete mathematics, hypergeometric functions and series expansions. It is defined by:
rf(x, k) = x * (x + 1) * ... * (x + k - 1)where ‘x’ can be arbitrary expression and ‘k’ is an integer. For more information check “Concrete mathematics” by Graham, pp. 66 or visit http://mathworld.wolfram.com/RisingFactorial.html page.
When x is a polynomial f of a single variable y of order >= 1, rf(x,k) = f(y) * f(y+1) * ... * f(x+k-1) as described in Peter Paule, “Greatest Factorial Factorization and Symbolic Summation”, Journal of Symbolic Computation, vol. 20, pp. 235-268, 1995.
See also
factorial,factorial2,FallingFactorialReferences
[R114] https://en.wikipedia.org/wiki/Pochhammer_symbol Examples
>>> from sympy import rf, symbols, factorial, ff, binomial >>> from sympy.abc import x >>> n, k = symbols('n k', integer=True) >>> rf(x, 0) 1 >>> rf(1, 5) 120 >>> rf(x, 5) == x*(1 + x)*(2 + x)*(3 + x)*(4 + x) True >>> rf(x**3, 2) Poly(x**6 + 3*x**5 + 3*x**4 + x**3, x, domain='ZZ')
Rewrite
>>> rf(x, k).rewrite(ff) FallingFactorial(k + x - 1, k) >>> rf(x, k).rewrite(binomial) binomial(k + x - 1, k)*factorial(k) >>> rf(n, k).rewrite(factorial) factorial(k + n - 1)/factorial(n - 1)
stirling¶
-
sympy.functions.combinatorial.numbers.stirling(n, k, d=None, kind=2, signed=False)[source]¶ Return Stirling number S(n, k) of the first or second (default) kind.
The sum of all Stirling numbers of the second kind for k = 1 through n is bell(n). The recurrence relationship for these numbers is:
{0} {n} {0} {n + 1} {n} { n } { } = 1; { } = { } = 0; { } = j*{ } + { } {0} {0} {k} { k } {k} {k - 1}
- where
jis:: nfor Stirling numbers of the first kind-nfor signed Stirling numbers of the first kindkfor Stirling numbers of the second kind
The first kind of Stirling number counts the number of permutations of
ndistinct items that havekcycles; the second kind counts the ways in whichndistinct items can be partitioned intokparts. Ifdis given, the “reduced Stirling number of the second kind” is returned:S^{d}(n, k) = S(n - d + 1, k - d + 1)withn >= k >= d. (This counts the ways to partitionnconsecutive integers intokgroups with no pairwise difference less thand. See example below.)To obtain the signed Stirling numbers of the first kind, use keyword
signed=True. Using this keyword automatically setskindto 1.References
[R115] http://en.wikipedia.org/wiki/Stirling_numbers_of_the_first_kind [R116] http://en.wikipedia.org/wiki/Stirling_numbers_of_the_second_kind Examples
>>> from sympy.functions.combinatorial.numbers import stirling, bell >>> from sympy.combinatorics import Permutation >>> from sympy.utilities.iterables import multiset_partitions, permutations
First kind (unsigned by default):
>>> [stirling(6, i, kind=1) for i in range(7)] [0, 120, 274, 225, 85, 15, 1] >>> perms = list(permutations(range(4))) >>> [sum(Permutation(p).cycles == i for p in perms) for i in range(5)] [0, 6, 11, 6, 1] >>> [stirling(4, i, kind=1) for i in range(5)] [0, 6, 11, 6, 1]
First kind (signed):
>>> [stirling(4, i, signed=True) for i in range(5)] [0, -6, 11, -6, 1]
Second kind:
>>> [stirling(10, i) for i in range(12)] [0, 1, 511, 9330, 34105, 42525, 22827, 5880, 750, 45, 1, 0] >>> sum(_) == bell(10) True >>> len(list(multiset_partitions(range(4), 2))) == stirling(4, 2) True
Reduced second kind:
>>> from sympy import subsets, oo >>> def delta(p): ... if len(p) == 1: ... return oo ... return min(abs(i[0] - i[1]) for i in subsets(p, 2)) >>> parts = multiset_partitions(range(5), 3) >>> d = 2 >>> sum(1 for p in parts if all(delta(i) >= d for i in p)) 7 >>> stirling(5, 3, 2) 7
- where
Enumeration¶
Three functions are available. Each of them attempts to efficiently compute
a given combinatorial quantity for a given set or multiset which can be
entered as an integer, sequence or multiset (dictionary with
elements as keys and multiplicities as values). The k parameter indicates
the number of elements to pick (or the number of partitions to make). When
k is None, the sum of the enumeration for all k (from 0 through the
number of items represented by n) is returned. A replacement parameter
is recognized for combinations and permutations; this indicates that any item
may appear with multiplicity as high as the number of items in the original
set.
>>> from sympy.functions.combinatorial.numbers import nC, nP, nT
>>> items = 'baby'
nC¶
Calculate the number of combinations of length k.
>>> [nC(items, k) for k in range(len(items) + 1)], nC(items)
([1, 3, 4, 3, 1], 12)
>>> nC('aaa', 2)
1
>>> nC('abc', 2)
3
>>> nC(3, 2)
3
nP¶
Calculate the number of permutations of length k.
>>> [nP(items, k) for k in range(len(items) + 1)], nP(items)
([1, 3, 7, 12, 12], 35)
>>> nC('aaa', 2)
1
>>> nC('abc', 2)
3
>>> nC(3, 2)
3
nT¶
Calculate the number of partitions that have k parts.
>>> [nT(items, k) for k in range(len(items) + 1)], nT(items)
([0, 1, 5, 4, 1], 11)
>>> nT('aaa', 2)
1
>>> nT('abc', 2)
3
>>> nT(3, 2)
1
Note that the integer for n indicates identical items for nT but
indicates n different items for nC and nP.
