17 Combinatorics

returns the factorial n! of the positive integer n, which is defined as the product 1 ·2 ·3 …n.

n! is the number of permutations of a set of n elements. 1/n! is the coefficient of xⁿ in the formal series e^x, which is the generating function for factorial.

gap> List( [0..10], Factorial );
[ 1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800 ]
gap> Factorial( 30 );
265252859812191058636308480000000

PermutationsList (see PermutationsList) computes the set of all permutations of a list.

returns the binomial coefficient

  n k  

of integers n and k, which is defined as n! / (k! (n−k)!) (see Factorial). We define

  0 0   = 1,   n k   = 0

if k < 0 or n < k, and

  n k   = (−1)k   −n+k−1 k  

if n < 0, which is consistent with the equivalent definition

  n k   =   n−1 k   +   n−1 k−1   ·

  n k  

is the number of combinations with k elements, i.e., the number of subsets with k elements, of a set with n elements.

  n k  

is the coefficient of the term x^k of the polynomial (x + 1)ⁿ, which is the generating function for

  n ·   ,

hence the name.

gap> List( [0..4], k->Binomial( 4, k ) );  # Knuth calls this the trademark of Binomial
[ 1, 4, 6, 4, 1 ]
gap> List( [0..6], n->List( [0..6], k->Binomial( n, k ) ) );;
gap> PrintArray( last );  # the lower triangle is called Pascal's triangle
[ [   1,   0,   0,   0,   0,   0,   0 ],
  [   1,   1,   0,   0,   0,   0,   0 ],
  [   1,   2,   1,   0,   0,   0,   0 ],
  [   1,   3,   3,   1,   0,   0,   0 ],
  [   1,   4,   6,   4,   1,   0,   0 ],
  [   1,   5,  10,  10,   5,   1,   0 ],
  [   1,   6,  15,  20,  15,   6,   1 ] ]
gap> Binomial( 50, 10 );
10272278170

NrCombinations (see Combinations) is the generalization of Binomial for multisets. Combinations (see Combinations) computes the set of all combinations of a multiset.

returns the Bell number B(n). The Bell numbers are defined by B(0)=1 and the recurrence

B(n+1) = n ∑ k=0    n k   B(k)

.

B(n) is the number of ways to partition a set of n elements into pairwise disjoint nonempty subsets (see PartitionsSet). This implies of course that B(n) = ∑_k=0ⁿS₂(n,k) (see Stirling2). B(n)/n! is the coefficient of xⁿ in the formal series e^ex−1, which is the generating function for B(n).

gap> List( [0..6], n -> Bell( n ) );
[ 1, 1, 2, 5, 15, 52, 203 ]
gap> Bell( 14 );
190899322

returns the n-th Bernoulli number B_n, which is defined by B₀ = 1 and

Bn = − n−1 ∑ k=0    n+1 k   Bk/(n+1)

.

B_n/n! is the coefficient of xⁿ in the power series of x/e^x−1. Except for B₁=−1/2 the Bernoulli numbers for odd indices are zero.

gap> Bernoulli( 4 );
-1/30
gap> Bernoulli( 10 );
5/66
gap> Bernoulli( 12 );  # there is no simple pattern in Bernoulli numbers
-691/2730
gap> Bernoulli( 50 );  # and they grow fairly fast
495057205241079648212477525/66

returns the Stirling number of the first kind S₁(n,k) of the integers n and k. Stirling numbers of the first kind are defined by S₁(0,0) = 1, S₁(n,0) = S₁(0,k) = 0 if n, k ≠ 0 and the recurrence S₁(n,k) = (n−1) S₁(n−1,k) + S₁(n−1,k−1).

S₁(n,k) is the number of permutations of n points with k cycles. Stirling numbers of the first kind appear as coefficients in the series

n!   x n   = n ∑ k=0  S1(n,k) xk

which is the generating function for Stirling numbers of the first kind. Note the similarity to

xn = n ∑ k=0  S2(n,k) k!   x k  

(see Stirling2). Also the definition of S₁ implies S₁(n,k) = S₂(−k,−n) if n,k < 0. There are many formulae relating Stirling numbers of the first kind to Stirling numbers of the second kind, Bell numbers, and Binomial coefficients.

gap> List( [0..4], k -> Stirling1( 4, k ) );  # Knuth calls this the trademark of S_1
[ 0, 6, 11, 6, 1 ]
gap> List( [0..6], n->List( [0..6], k->Stirling1( n, k ) ) );;
gap> # note the similarity with Pascal's triangle for the Binomial numbers
gap> PrintArray( last );
[ [    1,    0,    0,    0,    0,    0,    0 ],
  [    0,    1,    0,    0,    0,    0,    0 ],
  [    0,    1,    1,    0,    0,    0,    0 ],
  [    0,    2,    3,    1,    0,    0,    0 ],
  [    0,    6,   11,    6,    1,    0,    0 ],
  [    0,   24,   50,   35,   10,    1,    0 ],
  [    0,  120,  274,  225,   85,   15,    1 ] ]
gap> Stirling1(50,10);
101623020926367490059043797119309944043405505380503665627365376

returns the Stirling number of the second kind S₂(n,k) of the integers n and k. Stirling numbers of the second kind are defined by S₂(0,0) = 1, S₂(n,0) = S₂(0,k) = 0 if n, k ≠ 0 and the recurrence S₂(n,k) = k S₂(n−1,k) + S₂(n−1,k−1).

S₂(n,k) is the number of ways to partition a set of n elements into k pairwise disjoint nonempty subsets (see PartitionsSet). Stirling numbers of the second kind appear as coefficients in the expansion of

xn = n ∑ k=0  S2(n,k) k!   x k  

. Note the similarity to

n!   x n   = n ∑ k=0  S1(n,k) xk

(see Stirling1). Also the definition of S₂ implies S₂(n,k) = S₁(−k,−n) if n,k < 0. There are many formulae relating Stirling numbers of the second kind to Stirling numbers of the first kind, Bell numbers, and Binomial coefficients.

gap> List( [0..4], k->Stirling2( 4, k ) );  # Knuth calls this the trademark of S_2
[ 0, 1, 7, 6, 1 ]
gap> List( [0..6], n->List( [0..6], k->Stirling2( n, k ) ) );;
gap> # note the similarity with Pascal's triangle for the Binomial numbers
gap> PrintArray( last );
[ [   1,   0,   0,   0,   0,   0,   0 ],
  [   0,   1,   0,   0,   0,   0,   0 ],
  [   0,   1,   1,   0,   0,   0,   0 ],
  [   0,   1,   3,   1,   0,   0,   0 ],
  [   0,   1,   7,   6,   1,   0,   0 ],
  [   0,   1,  15,  25,  10,   1,   0 ],
  [   0,   1,  31,  90,  65,  15,   1 ] ]
gap> Stirling2( 50, 10 );
26154716515862881292012777396577993781727011

17.2 Combinations, Arrangements and Tuples

returns the set of all combinations of the multiset mset (a list of objects which may contain the same object several times) with k elements; if k is not given it returns all combinations of mset.

A combination of mset is an unordered selection without repetitions and is represented by a sorted sublist of mset. If mset is a proper set, there are

  |mset| k  

(see Binomial) combinations with k elements, and the set of all combinations is just the powerset of mset, which contains all subsets of mset and has cardinality 2^|mset|.

returns the number of Combinations(mset,k).

gap> Combinations( [1,2,2,3] );
[ [  ], [ 1 ], [ 1, 2 ], [ 1, 2, 2 ], [ 1, 2, 2, 3 ], [ 1, 2, 3 ], [ 1, 3 ], 
  [ 2 ], [ 2, 2 ], [ 2, 2, 3 ], [ 2, 3 ], [ 3 ] ]
gap> NrCombinations( [1..52], 5 );  # number of different hands in a game of poker 
2598960

The function Arrangements (see Arrangements) computes ordered selections without repetitions, UnorderedTuples (see UnorderedTuples) computes unordered selections with repetitions and Tuples (see Tuples) computes ordered selections with repetitions.

returns the set of arrangements of the multiset mset that contain k elements. If k is not given it returns all arrangements of mset.

An arrangement of mset is an ordered selection without repetitions and is represented by a list that contains only elements from mset, but maybe in a different order. If mset is a proper set there are |mset|! / (|mset|−k)! (see Factorial) arrangements with k elements.

returns the number of Arrangements(mset,k).

As an example of arrangements of a multiset, think of the game Scrabble. Suppose you have the six characters of the word settle and you have to make a four letter word. Then the possibilities are given by

gap> Arrangements( ["s","e","t","t","l","e"], 4 );
[ [ "e", "e", "l", "s" ], [ "e", "e", "l", "t" ], [ "e", "e", "s", "l" ], 
  [ "e", "e", "s", "t" ], [ "e", "e", "t", "l" ], [ "e", "e", "t", "s" ], 
  ... 93 more possibilities ...
  [ "t", "t", "l", "s" ], [ "t", "t", "s", "e" ], [ "t", "t", "s", "l" ] ]

Can you find the five proper English words, where lets does not count? Note that the fact that the list returned by Arrangements is a proper set means in this example that the possibilities are listed in the same order as they appear in the dictionary.

gap> NrArrangements( ["s","e","t","t","l","e"] );
523

The function Combinations (see Combinations) computes unordered selections without repetitions, UnorderedTuples (see UnorderedTuples) computes unordered selections with repetitions and Tuples (see Tuples) computes ordered selections with repetitions.

returns the set of all unordered tuples of length k of the set set.

An unordered tuple of length k of set is a unordered selection with repetitions of set and is represented by a sorted list of length k containing elements from set. There are

  |set|+k−1 k  

(see Binomial) such unordered tuples.

Note that the fact that UnorderedTuples returns a set implies that the last index runs fastest. That means the first tuple contains the smallest element from set k times, the second tuple contains the smallest element of set at all positions except at the last positions, where it contains the second smallest element from set and so on.

returns the number of UnorderedTuples(set,k).

As an example for unordered tuples think of a poker-like game played with 5 dice. Then each possible hand corresponds to an unordered five-tuple from the set [1..6]

gap> NrUnorderedTuples( [1..6], 5 );
252
gap> UnorderedTuples( [1..6], 5 );
[ [ 1, 1, 1, 1, 1 ], [ 1, 1, 1, 1, 2 ], [ 1, 1, 1, 1, 3 ], [ 1, 1, 1, 1, 4 ], 
  [ 1, 1, 1, 1, 5 ], [ 1, 1, 1, 1, 6 ], [ 1, 1, 1, 2, 2 ], [ 1, 1, 1, 2, 3 ], 
  ... 100 more tuples ...
  [ 1, 3, 5, 5, 6 ], [ 1, 3, 5, 6, 6 ], [ 1, 3, 6, 6, 6 ], [ 1, 4, 4, 4, 4 ], 
  ... 100 more tuples ...
  [ 3, 3, 5, 5, 5 ], [ 3, 3, 5, 5, 6 ], [ 3, 3, 5, 6, 6 ], [ 3, 3, 6, 6, 6 ], 
  ... 32 more tuples ...
  [ 5, 5, 5, 6, 6 ], [ 5, 5, 6, 6, 6 ], [ 5, 6, 6, 6, 6 ], [ 6, 6, 6, 6, 6 ] ]

The function Combinations (see Combinations) computes unordered selections without repetitions, Arrangements (see Arrangements) computes ordered selections without repetitions and Tuples (see Tuples) computes ordered selections with repetitions.

returns the set of all ordered tuples of length k of the set set.

An ordered tuple of length k of set is an ordered selection with repetition and is represented by a list of length k containing elements of set. There are |set|^k such ordered tuples.

Note that the fact that Tuples returns a set implies that the last index runs fastest. That means the first tuple contains the smallest element from set k times, the second tuple contains the smallest element of set at all positions except at the last positions, where it contains the second smallest element from set and so on.

returns the number of Tuples(set,k).

gap> Tuples( [1,2,3], 2 );
[ [ 1, 1 ], [ 1, 2 ], [ 1, 3 ], [ 2, 1 ], [ 2, 2 ], [ 2, 3 ], [ 3, 1 ], 
  [ 3, 2 ], [ 3, 3 ] ]
gap> NrTuples( [1..10], 5 );
100000

Tuples(set,k) can also be viewed as the k-fold cartesian product of set (see Cartesian).

The function Combinations (see Combinations) computes unordered selections without repetitions, Arrangements (see Arrangements) computes ordered selections without repetitions, and finally the function UnorderedTuples (see UnorderedTuples) computes unordered selections with repetitions.

PermutationsList returns the set of permutations of the multiset mset.

A permutation is represented by a list that contains exactly the same elements as mset, but possibly in different order. If mset is a proper set there are |mset| ! (see Factorial) such permutations. Otherwise if the first elements appears k₁ times, the second element appears k₂ times and so on, the number of permutations is |mset|! / (k₁! k₂! …), which is sometimes called multinomial coefficient.

returns the number of PermutationsList(mset).

gap> PermutationsList( [1,2,3] );
[ [ 1, 2, 3 ], [ 1, 3, 2 ], [ 2, 1, 3 ], [ 2, 3, 1 ], [ 3, 1, 2 ], 
  [ 3, 2, 1 ] ]
gap> PermutationsList( [1,1,2,2] );
[ [ 1, 1, 2, 2 ], [ 1, 2, 1, 2 ], [ 1, 2, 2, 1 ], [ 2, 1, 1, 2 ], 
  [ 2, 1, 2, 1 ], [ 2, 2, 1, 1 ] ]
gap> NrPermutationsList( [1,2,2,3,3,3,4,4,4,4] );
12600

The function Arrangements (see Arrangements) is the generalization of PermutationsList that allows you to specify the size of the permutations. Derangements (see Derangements) computes permutations that have no fixpoints.

returns the set of all derangements of the list list.

A derangement is a fixpointfree permutation of list and is represented by a list that contains exactly the same elements as list, but in such an order that the derangement has at no position the same element as list. If the list list contains no element twice there are exactly |list|! (1/2! − 1/3! + 1/4! − …+ (−1)ⁿ/n!) derangements.

Note that the ratio NrPermutationsList([1..n])/NrDerangements([1..n]), which is n! / (n! (1/2! − 1/3! + 1/4! − …+ (−1)ⁿ/n!)) is an approximation for the base of the natural logarithm e = 2.7182818285…, which is correct to about n digits.

returns the number of Derangements(list).

As an example of derangements suppose that you have to send four different letters to four different people. Then a derangement corresponds to a way to send those letters such that no letter reaches the intended person.

gap> Derangements( [1,2,3,4] );
[ [ 2, 1, 4, 3 ], [ 2, 3, 4, 1 ], [ 2, 4, 1, 3 ], [ 3, 1, 4, 2 ], 
  [ 3, 4, 1, 2 ], [ 3, 4, 2, 1 ], [ 4, 1, 2, 3 ], [ 4, 3, 1, 2 ], 
  [ 4, 3, 2, 1 ] ]
gap> NrDerangements( [1..10] );
1334961
gap> Int( 10^7*NrPermutationsList([1..10])/last );
27182816
gap> Derangements( [1,1,2,2,3,3] );
[ [ 2, 2, 3, 3, 1, 1 ], [ 2, 3, 1, 3, 1, 2 ], [ 2, 3, 1, 3, 2, 1 ], 
  [ 2, 3, 3, 1, 1, 2 ], [ 2, 3, 3, 1, 2, 1 ], [ 3, 2, 1, 3, 1, 2 ], 
  [ 3, 2, 1, 3, 2, 1 ], [ 3, 2, 3, 1, 1, 2 ], [ 3, 2, 3, 1, 2, 1 ], 
  [ 3, 3, 1, 1, 2, 2 ] ]
gap> NrDerangements( [1,2,2,3,3,3,4,4,4,4] );
338

The function PermutationsList (see PermutationsList) computes all permutations of a list.

returns the set of all unordered partitions of the set set into k pairwise disjoint nonempty sets. If k is not given it returns all unordered partitions of set for all k.

An unordered partition of set is a set of pairwise disjoint nonempty sets with union set and is represented by a sorted list of such sets. There are B( |set| ) (see Bell) partitions of the set set and S₂( |set|, k ) (see Stirling2) partitions with k elements.

returns the number of PartitionsSet(set,k).

gap> PartitionsSet( [1,2,3] );
[ [ [ 1 ], [ 2 ], [ 3 ] ], [ [ 1 ], [ 2, 3 ] ], [ [ 1, 2 ], [ 3 ] ], 
  [ [ 1, 2, 3 ] ], [ [ 1, 3 ], [ 2 ] ] ]
gap> PartitionsSet( [1,2,3,4], 2 );
[ [ [ 1 ], [ 2, 3, 4 ] ], [ [ 1, 2 ], [ 3, 4 ] ], [ [ 1, 2, 3 ], [ 4 ] ], 
  [ [ 1, 2, 4 ], [ 3 ] ], [ [ 1, 3 ], [ 2, 4 ] ], [ [ 1, 3, 4 ], [ 2 ] ], 
  [ [ 1, 4 ], [ 2, 3 ] ] ]
gap> NrPartitionsSet( [1..6] );
203
gap> NrPartitionsSet( [1..10], 3 );
9330

Note that PartitionsSet does currently not support multisets and that there is currently no ordered counterpart.

returns the set of all (unordered) partitions of the positive integer n into sums with k summands. If k is not given it returns all unordered partitions of set for all k.

An unordered partition is an unordered sum n = p₁+p₂ +…+ p_k of positive integers and is represented by the list p = [p₁,p₂,…,p_k], in nonincreasing order, i.e., p₁ > =p₂ > = … > =p_k. We write p |- n. There are approximately e^{π√{2/3 n}} / 4 √3 n such partitions.

It is possible to associate with every partition of the integer n a conjugacy class of permutations in the symmetric group on n points and vice versa. Therefore p(n) : = NrPartitions(n) is the number of conjugacy classes of the symmetric group on n points.

Ramanujan found the identities p(5i+4) = 0 mod 5, p(7i+5) = 0 mod 7 and p(11i+6) = 0 mod 11 and many other fascinating things about the number of partitions.

Do not call Partitions with an n much larger than 40, in which case there are 37338 partitions, since the list will simply become too large.

returns the number of Partitions(set,k).

gap> Partitions( 7 );
[ [ 1, 1, 1, 1, 1, 1, 1 ], [ 2, 1, 1, 1, 1, 1 ], [ 2, 2, 1, 1, 1 ], 
  [ 2, 2, 2, 1 ], [ 3, 1, 1, 1, 1 ], [ 3, 2, 1, 1 ], [ 3, 2, 2 ], 
  [ 3, 3, 1 ], [ 4, 1, 1, 1 ], [ 4, 2, 1 ], [ 4, 3 ], [ 5, 1, 1 ], [ 5, 2 ], 
  [ 6, 1 ], [ 7 ] ]
gap> Partitions( 8, 3 );
[ [ 3, 3, 2 ], [ 4, 2, 2 ], [ 4, 3, 1 ], [ 5, 2, 1 ], [ 6, 1, 1 ] ]
gap> NrPartitions( 7 );
15
gap> NrPartitions( 100 );
190569292

The function OrderedPartitions (see OrderedPartitions) is the ordered counterpart of Partitions.

returns the set of all ordered partitions of the positive integer n into sums with k summands. If k is not given it returns all ordered partitions of set for all k.

An ordered partition is an ordered sum n = p₁ + p₂ +…+ p_k of positive integers and is represented by the list [ p₁, p₂,…, p_k ]. There are totally 2ⁿ⁻¹ ordered partitions and

  n−1 k−1  

(see Binomial) ordered partitions with k summands.

Do not call OrderedPartitions with an n much larger than 15, the list will simply become too large.

returns the number of OrderedPartitions(set,k).

gap> OrderedPartitions( 5 );
[ [ 1, 1, 1, 1, 1 ], [ 1, 1, 1, 2 ], [ 1, 1, 2, 1 ], [ 1, 1, 3 ], 
  [ 1, 2, 1, 1 ], [ 1, 2, 2 ], [ 1, 3, 1 ], [ 1, 4 ], [ 2, 1, 1, 1 ], 
  [ 2, 1, 2 ], [ 2, 2, 1 ], [ 2, 3 ], [ 3, 1, 1 ], [ 3, 2 ], [ 4, 1 ], [ 5 ] ]
gap> OrderedPartitions( 6, 3 );
[ [ 1, 1, 4 ], [ 1, 2, 3 ], [ 1, 3, 2 ], [ 1, 4, 1 ], [ 2, 1, 3 ], 
  [ 2, 2, 2 ], [ 2, 3, 1 ], [ 3, 1, 2 ], [ 3, 2, 1 ], [ 4, 1, 1 ] ]
gap> NrOrderedPartitions(20);
524288

The function Partitions (see Partitions) is the unordered counterpart of OrderedPartitions.

returns the set of all (unordered) partitions of the integer n having parts less or equal to the integer m.

returns the set of all (unordered) partitions of the integer n having greatest part equal to the integer m.

In the first form RestrictedPartitions returns the set of all restricted partitions of the positive integer n into sums with k summands with the summands of the partition coming from the set set. If k is not given all restricted partitions for all k are returned.

A restricted partition is like an ordinary partition (see Partitions) an unordered sum n = p₁+p₂+…+p_k of positive integers and is represented by the list p = [p₁,p₂,…,p_k], in nonincreasing order. The difference is that here the p_i must be elements from the set set, while for ordinary partitions they may be elements from [1..n].

returns the number of RestrictedPartitions(n,set,k).

gap> RestrictedPartitions( 8, [1,3,5,7] );
[ [ 1, 1, 1, 1, 1, 1, 1, 1 ], [ 3, 1, 1, 1, 1, 1 ], [ 3, 3, 1, 1 ], 
  [ 5, 1, 1, 1 ], [ 5, 3 ], [ 7, 1 ] ]
gap> NrRestrictedPartitions(50,[1,2,5,10,20,50]);
451

The last example tells us that there are 451 ways to return 50 pence change using 1,2,5,10,20 and 50 pence coins.

returns the sign of a permutation with cycle structure pi.

This function actually describes a homomorphism from the symmetric group S_n into the cyclic group of order 2, whose kernel is exactly the alternating group A_n (see SignPerm). Partitions of sign 1 are called even partitions while partitions of sign −1 are called odd.

gap> SignPartition([6,5,4,3,2,1]);
-1

AssociatedPartition returns the associated partition of the partition pi which is obtained by transposing the corresponding Young diagram.

gap> AssociatedPartition([4,2,1]);
[ 3, 2, 1, 1 ]
gap> AssociatedPartition([6]);
[ 1, 1, 1, 1, 1, 1 ]

PowerPartition returns the partition corresponding to the k-th power of a permutation with cycle structure pi.

Each part l of pi is replaced by d = gcd(l, k) parts l/d. So if pi is a partition of n then pi ^k also is a partition of n. PowerPartition describes the powermap of symmetric groups.

gap> PowerPartition([6,5,4,3,2,1], 3);
[ 5, 4, 2, 2, 2, 2, 1, 1, 1, 1 ]

PartitionTuples returns the list of all r-tuples of partitions which together form a partition of n.

r--tuples of partitions describe the classes and the characters of wreath products of groups with r conjugacy classes with the symmetric group S_n.

returns the number of PartitionTuples( n, r ).

gap> PartitionTuples(3, 2);
[ [ [ 1, 1, 1 ], [  ] ], [ [ 1, 1 ], [ 1 ] ], [ [ 1 ], [ 1, 1 ] ], 
  [ [  ], [ 1, 1, 1 ] ], [ [ 2, 1 ], [  ] ], [ [ 1 ], [ 2 ] ], 
  [ [ 2 ], [ 1 ] ], [ [  ], [ 2, 1 ] ], [ [ 3 ], [  ] ], [ [  ], [ 3 ] ] ]

17.3 Fibonacci and Lucas Sequences

returns the nth number of the Fibonacci sequence. The Fibonacci sequence F_n is defined by the initial conditions F₁=F₂=1 and the recurrence relation F_n+2 = F_n+1 + F_n. For negative n we define F_n = (−1)ⁿ⁺¹ F_−n, which is consistent with the recurrence relation.

Using generating functions one can prove that F_n = φⁿ − 1/φⁿ, where φ is (√5 + 1)/2, i.e., one root of x² − x − 1 = 0. Fibonacci numbers have the property Gcd( F_m, F_n ) = F_Gcd(m,n). But a pair of Fibonacci numbers requires more division steps in Euclid's algorithm (see Gcd) than any other pair of integers of the same size. Fibonacci(k) is the special case Lucas(1,-1,k)[1] (see Lucas).

gap> Fibonacci( 10 );
55
gap> Fibonacci( 35 );
9227465
gap> Fibonacci( -10 );
-55

returns the k-th values of the Lucas sequence with parameters P and Q, which must be integers, as a list of three integers.

Let α, β be the two roots of x² − P x + Q then we define Lucas( P, Q, k )[1] = U_k = (α^k − β^k) / (α− β) and Lucas( P, Q, k )[2] = V_k = (α^k + β^k) and as a convenience Lucas( P, Q, k )[3] = Q^k.

The following recurrence relations are easily derived from the definition U₀ = 0, U₁ = 1, U_k = P U_k−1 − Q U_k−2 and V₀ = 2, V₁ = P, V_k = P V_k−1 − Q V_k−2. Those relations are actually used to define Lucas if α = β.

Also the more complex relations used in Lucas can be easily derived U_2k = U_k V_k, U_2k+1 = (P U_2k + V_2k) / 2 and V_2k = V_k² − 2 Q^k, V_2k+1 = ((P²−4Q) U_2k + P V_2k) / 2.

Fibonacci(k) (see Fibonacci) is simply Lucas(1,-1,k)[1]. In an abuse of notation, the sequence Lucas(1,-1,k)[2] is sometimes called the Lucas sequence.

gap> List( [0..10], i -> Lucas(1,-2,i)[1] );     # 2^k - (-1)^k)/3
[ 0, 1, 1, 3, 5, 11, 21, 43, 85, 171, 341 ]
gap> List( [0..10], i -> Lucas(1,-2,i)[2] );     # 2^k + (-1)^k
[ 2, 1, 5, 7, 17, 31, 65, 127, 257, 511, 1025 ]
gap> List( [0..10], i -> Lucas(1,-1,i)[1] );     # Fibonacci sequence
[ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 ]
gap> List( [0..10], i -> Lucas(2,1,i)[1] );      # the roots are equal 
[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

17.4 Permanent of a Matrix

returns the permanent of the matrix mat. The permanent is defined by ∑_{p ∈ Symm(n)}∏_i=1ⁿmat[i][i^p].

Note the similarity of the definition of the permanent to the definition of the determinant (see DeterminantMat). In fact the only difference is the missing sign of the permutation. However the permanent is quite unlike the determinant, for example it is not multilinear or alternating. It has however important combinatorial properties.

gap> Permanent( [[0,1,1,1],
>      [1,0,1,1],
>      [1,1,0,1],
>      [1,1,1,0]] );  # inefficient way to compute `NrDerangements([1..4])'
9
gap> Permanent( [[1,1,0,1,0,0,0],
>      [0,1,1,0,1,0,0],
>      [0,0,1,1,0,1,0],
>      [0,0,0,1,1,0,1],
>      [1,0,0,0,1,1,0],
>      [0,1,0,0,0,1,1],
>      [1,0,1,0,0,0,1]] );  # 24 permutations fit the projective plane of order 2
24

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GAP 4 manual
March 2006