Compute the Eulerian number

Question

The Eulerian number A(n, m) is the number of permutations of [1, 2, ..., n] in which exactly m elements are greater than the previous element. These are also called rises. For example, if n = 3, there are 3! = 6 permutations of [1, 2, 3]

1 2 3
 < <  2 elements are greater than the previous

1 3 2
 < >  1 ...

2 1 3
 > <  1 ...

2 3 1
 < >  1 ...

3 1 2
 > <  1 ...

3 2 1
 > >  0 ...

So the outputs for A(3, m) for m in [0, 1, 2, 3] will be

A(3, 0) = 1
A(3, 1) = 4
A(3, 2) = 1
A(3, 3) = 0

Also, this is the OEIS sequence A173018.

Rules

• This is code-golf so the shortest code wins.
• The input n will be a nonnegative integer and m will be a integer in the range [0, 1, ..., n].

Test Cases

n   m   A(n, m)
0   0   1
1   0   1
1   1   0
2   0   1
2   1   1
2   2   0
3   0   1
3   1   4
3   2   1
3   3   0
4   0   1
4   1   11
4   2   11
4   3   1
4   4   0
5   1   26
7   4   1191
9   5   88234
10  5   1310354
10  7   47840
10  10  0
12  2   478271
15  6   311387598411
17  1   131054
20  16  1026509354985
42  42  0

Answer 1

Jelly, 8 bytes

Œ!Z>2\Sċ

Try it online! (takes a while) or verify the smaller test cases.

How it works

Œ!Z>2\Sċ  Main link. Arguments: n, m

Œ!        Generate the matrix of all permutations of [1, ..., n].
  Z       Zip/transpose, placing the permutations in the columns.
   >2\    Compare columns pairwise with vectorizing greater-than.
          This generates a 1 in the column for each rise in that permutation.
      S   Compute the vectorizing sum of the columns, counting the number of rises.
       ċ  Count how many times m appears in the computed counts.

Answer 2

JavaScript (ES6), 50 46 45 bytes

f=(n,m,d=n-m)=>m?d&&f(--n,m)*++m+f(n,m-2)*d:1

Based on the recursive formula:

A(n, m) = (n - m)A(n - 1, m - 1) + (m + 1)A(n - 1, m)    

Test cases

f=(n,m,d=n-m)=>m?d&&f(--n,m)*++m+f(n,m-2)*d:1

console.log(f( 0,  0));  // 1
console.log(f( 1,  0));  // 1
console.log(f( 1,  1));  // 0
console.log(f( 2,  0));  // 1
console.log(f( 2,  1));  // 1
console.log(f( 2,  2));  // 0
console.log(f( 3,  0));  // 1
console.log(f( 3,  1));  // 4
console.log(f( 3,  2));  // 1
console.log(f( 3,  3));  // 0
console.log(f( 4,  0));  // 1
console.log(f( 4,  1));  // 11
console.log(f( 4,  2));  // 11
console.log(f( 4,  3));  // 1
console.log(f( 4,  4));  // 0
console.log(f( 5,  1));  // 26
console.log(f( 7,  4));  // 1191
console.log(f( 9,  5));  // 88234
console.log(f(10,  5));  // 1310354
console.log(f(10,  7));  // 47840
console.log(f(10, 10));  // 0
console.log(f(12,  2));  // 478271
console.log(f(15,  6));  // 311387598411
console.log(f(17,  1));  // 131054
console.log(f(20, 16));  // 1026509354985
console.log(f(42, 42));  // 0

Answer 3

MATL, 10 bytes

:Y@!d0>s=s

Try it online!

Explanation

Consider as an example inputs n=3, m=1. You can place a % symbol to comment out the code from that point onwards and thus see the intermediate results. For example, the link shows the stack after the first step.

:      % Input n implicitly. Push [1 2 ... n]
       % STACK: [1 2 ... n]
Y@     % Matrix of all permutations, one on each row
       % STACK: [1 2 3; 1 3 2; 2 1 3; 2 3 1; 3 1 2; 3 2 1]
!      % Transpose
       % STACK: [1 1 2 2 3 3; 2 3 1 3 1 2; 3 2 3 1 2 1]
d      % Consecutive differences along each column
       % STACK: [1 2 -1 1 -2 -1; 1 -1 2 -2 1 -1]
0>     % True for positive entries
       % STACK: [1 1 0 1 0 0; 1 0 1 0 1 0]
s      % Sum of each column
       % STACK: [2 1 1 1 1 0]
=      % Input m implicitly. Test each entry for equality with m
       % STACK: [0 1 1 1 1 0]
s      % Sum. Implicitly display
       % STACK: 4

Answer 4

CJam (21 19 bytes - or 18 if argument order is free)

{\e!f{2ew::>1b=}1b}

This is an anonymous block (function) which takes n m on the stack. (If it's permitted to take m n on the stack then the \ can be saved). It computes all permutations and filters, so the online test suite must be rather limited.

Thanks to Martin for pointing out an approximation to filter-with-parameter.

Dissection

{        e# Define a block. Stack: n m
  \      e#   Flip the stack to give m n
  e!f{   e#   Generate permutations of [0 .. n-1] and map with parameter m
    2ew  e#     Stack: m perm; generate the list of n-1 pairs of consecutive
         e#     elements of perm
    ::>  e#     Map each pair to 1 if it's a rise and 0 if it's a fall
    1b   e#     Count the falls
    =    e#     Map to 1 if there are m falls and 0 otherwise
  }
  1b     e#   Count the permutations with m falls
}

Note that the Eulerian numbers are symmetric: E(n, m) = E(n, n-m), so it's irrelevant whether you count falls or rises.

Efficiently: 32 bytes

{1a@{0\+_ee::*(;\W%ee::*W%.+}*=}

Online test suite.

This implements the recurrence on whole rows.

{          e# Define a block. Stack: n m
  1a@      e#   Push the row for n=0: [1]; and rotate n to top of stack
  {        e#   Repeat n times:
           e#     Stack: m previous-row
    0\+_   e#     Prepend a 0 to the row and duplicate
    ee::*  e#     Multiply each element by its index
           e#     This gives A[j] = j * E(i-1, j-1)
    (;     e#     Pop the first element, so that A[j] = (j+1) * E(i-1, j)
    \W%    e#     Get the other copy of the previous row and reverse it
    ee::*  e#     Multiply each element by its index
           e#     This gives B[j] = j * E(i-1, i-1-j)
    W%     e#     Reverse again, giving B[j] = (i-j) * E(i-1, j-1)
    .+     e#     Pointwise addition
  }*
  =        e#   Extract the element at index j
}

Answer 5

Python, 55 56 bytes

a=lambda n,m:n>=m>0and(n-m)*a(n-1,m-1)-~m*a(n-1,m)or m<1

All tests at repl.it

Applies the recursive formula on OEIS.
Note that +(m+1)*a(n-1,m) is golfed to -~m*a(n-1,m).
(May return boolean values to represent 1 or 0. Returns True when n<0 and m<=0 or m<0.)

Answer 6

Mathematica, 59 56 bytes

_~f~0=1
n_~f~m_:=If[m>n,0,(n-m)f[n-1,m-1]+(m+1)f[n-1,m]]

And here is a 59 byte version implementing the definition more literally:

Count[Count@1/@Sign/@Differences/@Permutations@Range@#,#2]&

Answer 7

Python, 53 bytes

t=lambda n,k:n and(n-k)*t(n-1,k-1)-~k*t(n-1,k)or k==0

Recursion from OEIS. Outputs Boolean True as 1 when n==k.

Answer 8

MATLAB / Octave, 40 bytes

@(n,m)sum(sum(diff((perms(1:n))')>0)==m)

This is a port of my MATL answer, in the form of an anonymous function. Call it as ans(7,4).

Try it at Ideone.

Answer 9

GameMaker Language, 62 bytes

This is a recursive script A based on @Arnauld's formula.

n=argument0;m=argument1;return (n-m)*A(n-1,m-1)+(m+1)*A(n-1,m)

Answer 10

Perl, 98 bytes

sub a{my($b,$c)=@_;return$c?$c>$b?0:($b-$c)*a($b-1,$c-1)+($c+1)*a($b-1,$c):1;}print a(@ARGV[0,1]);

Based on the same property as Arnauld's answer.

Answer 11

R, 72 bytes

Recursive function following the logic on OEIS.

A=function(n,m)if(!m)1 else if(m-n)0 else(n-m)*A(n-1,m-1)+(m+1)*A(n-1,m)

This challenge turned out to be quite close between the different approaches I tried. For instance, using the wikipedia formula and looping over the sum resulted in 92 bytes:

function(n,m){s=0;if(!n)1 else for(k in -1:m+1)s=c(s,(-1)^k*choose(n+1,k)*(m+1-k)^n);sum(s)}

or the vectorized version for 87 bytes:

function(n,m)if(!m)1 else sum(sapply(-1:m+1,function(k)(-1)^k*choose(n+1,k)*(m+1-k)^n))

and finally the brute force solution (103 bytes) that generates a matrix of all permutations by using the permute package and the function allPerms. This approach only works up to n<8 though.

function(n,m){if(!m)1 else sum(apply(rbind(1:n,permute:::allPerms(n)),1,function(x)sum(diff(x)>0))==m)}

Answer 12

Racket 141 bytes

(count(λ(x)(= x m))(for/list((t(permutations(range 1(+ 1 n)))))(count
(λ(x)x)(for/list((i(sub1 n)))(>(list-ref t(+ 1 i))(list-ref t i))))))

Ungolfed:

(define (f n m)
  (let* ((l (range 1 (add1 n)))                ; create a list till n
         (pl (permutations l))                 ; get all permutations
         (enl (for/list ((t pl))               ; check each permutation; 
                (define rl
                  (for/list ((i (sub1 n)))     ; check if an element is a 'rise'
                    (> (list-ref t (add1 i))
                       (list-ref t i))))
                (count (lambda(x)x) rl))))     ; how many numbers are 'rises'
    (count (lambda(x) (= x m)) enl)))          ; how many permutations had m rises
                                               ; i.e. Eulerian number

Testing:

(f 3 0)
(f 3 1)
(f 3 2)
(f 3 3)
(f 4 2)
(f 5 1)
(f 7 4)

Output:

1
4
1
0
11
26
1191

Answer 13

Actually, 21 19 bytes

This answer uses an algorithm similar to the one Dennis uses in his Jelly answer. The original definition counts < while I count >. This ends up being equivalent in the end. Golfing suggestions welcome. Try it online!

;R╨`;\ZdX"i>"£MΣ`Mc

Ungolfing

         Implicit input m, then n.
;        Duplicate n. Stack: n, n, m
R        Push range [1..n].
╨        Push all n-length permutations of the range.
`...`M   Map the following function over each permutation p.
  ;\       Duplicate and rotate p so that we have a list of the next elements of p.
  Z        Zip rot_p and p.
           (order of operands here means the next element is first,
            so we need to use > later)
  dX       Remove the last pair as we don't compare the last and first elements of the list.
  "i>"£    Create a function that will flatten a list and check for a rise.
  M        Map that function over all the pairs.
  Σ        Count how many rises there are in each permutation.
c        Using the result of the map and the remaining m, 
          count how many permutations have m rises.
         Implicit return.

Answer 14

Swift 3, 82 88 Bytes

func A(_ n:Int,_ m:Int)->Int{return m<1 ?1:n>m ?(n-m)*A(n-1,m-1)+(m+1)*A(n-1,m):0}

Just the same recursive formula in a language that is definitely not for golf

Answer 15

J, 28 bytes

+/@((!>:)~*(^~#\.)*_1^])i.,]

Uses the formula

Usage

   f =: +/@((!>:)~*(^~#\.)*_1^])i.,]
   0 f 0
1
   1 f 0
1
   1 f 1
0
   (f"+i.,]) 6
1 57 302 302 57 1 0
   20x f 16x
1026509354985

Explanation

+/@((!>:)~*(^~#\.)*_1^])i.,]  Input: n (LHS), m (RHS)
                        i.    Range [0, 1, ..., m-1]
                           ]  Get m
                          ,   Join to get k = [0, 1, ..., m]
                      ]       Get k
                   _1^        Raise -1 to each in k
              #\.               Get the length of each suffix of k
                                Forms the range [m+1, m, ..., 2, 1]
            ^~                  Raise each value by n
                  *           Multiply elementwise with (-1)^k
    (   )~                      Commute operators
      >:                        Increment n
     !                          Binomial coefficient, C(n+1, k)
          *                   Multiply elementwise
+/@                           Reduce by addition to get the sum and return