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{{Built-in|Binomial|!}} is a [[dyadic]] [[scalar function]] which gives the [[wikipedia:binomial coefficient|binomial coefficient]] <math>\tbinom nk</math> between the two [[argument|arguments]]. The argument order <source lang=apl inline>k!n</source> is reversed compared to most of traditional mathematical notation's alternative notations, for example <math>C(n,k)</math> and <math>_nC_k</math>, but not others, like <math>C_n^k</math>. Binomial shares the [[glyph]] <source lang=apl inline>!</source> with the monadic arithmetic function [[Factorial]].
{{Built-in|Binomial|!}} is a [[dyadic]] [[scalar function]] which gives the [[wikipedia:binomial coefficient|binomial coefficient]] <math>\tbinom nk</math> between the two [[argument|arguments]]. The argument order <syntaxhighlight lang=apl inline>k!n</syntaxhighlight> is reversed compared to most of traditional mathematical notation's alternative notations, for example <math>C(n,k)</math> and <math>_nC_k</math>, but not others, like <math>C_n^k</math>. Binomial shares the [[glyph]] <syntaxhighlight lang=apl inline>!</syntaxhighlight> with the monadic arithmetic function [[Factorial]].


== Examples ==
== Examples ==


For non-negative integer arguments, the binomial coefficient <source lang=apl inline>k!n</source> is equal to the number of ways to choose k items out of n distinct items. For example, <source lang=apl inline>3!5</source> is 10 because there are 10 ways to pick 3 items out of 5: 123, 124, 125, 134, 135, 145, 234, 235, 245, 345.
For non-negative integer arguments, the binomial coefficient <syntaxhighlight lang=apl inline>k!n</syntaxhighlight> is equal to the number of ways to choose k items out of n distinct items. For example, <syntaxhighlight lang=apl inline>3!5</syntaxhighlight> is 10 because there are 10 ways to pick 3 items out of 5: 123, 124, 125, 134, 135, 145, 234, 235, 245, 345.


<source lang=apl>
<syntaxhighlight lang=apl>
       0 1 2 3 4 5!5
       0 1 2 3 4 5!5
1 5 10 10 5 1
1 5 10 10 5 1
</source>
</syntaxhighlight>


<source lang=apl inline>k!n</source> also corresponds to the k-th value (zero-indexed) on the n-th row (also zero-indexed) of [[wikipedia:Pascal's triangle|Pascal's triangle]].
<syntaxhighlight lang=apl inline>k!n</syntaxhighlight> also corresponds to the k-th value (zero-indexed) on the n-th row (also zero-indexed) of [[wikipedia:Pascal's triangle|Pascal's triangle]].


<source lang=apl>
<syntaxhighlight lang=apl>
       ⍉∘.!⍨ 0,⍳5
       ⍉∘.!⍨ 0,⍳5
1 0  0  0 0 0
1 0  0  0 0 0
Line 20: Line 20:
1 4  6  4 1 0
1 4  6  4 1 0
1 5 10 10 5 1
1 5 10 10 5 1
</source>{{Works in|[[Dyalog APL]]}}
</syntaxhighlight>{{Works in|[[Dyalog APL]]}}


== Properties ==
== Properties ==


The value of <source lang=apl inline>X!Y</source> equals <source lang=apl inline>(!Y)÷(!X)×!Y-X</source>.
The value of <syntaxhighlight lang=apl inline>X!Y</syntaxhighlight> equals <syntaxhighlight lang=apl inline>(!Y)÷(!X)×!Y-X</syntaxhighlight>.


<source lang=apl>
<syntaxhighlight lang=apl>
       Alt←{(!⍵)÷(!⍺)×!⍵-⍺}
       Alt←{(!⍵)÷(!⍺)×!⍵-⍺}
       0 1 2 3 4 5 Alt 5
       0 1 2 3 4 5 Alt 5
1 5 10 10 5 1
1 5 10 10 5 1
</source>
</syntaxhighlight>


In multiple implementations where [[Factorial]] is extended to use the [[wikipedia:Gamma function|Gamma function]] <math>\Gamma(n)</math>, Binomial is defined to use the above equality for non-integers. In that case, the [[wikipedia:Beta function|Beta function]] <math>\Beta(x,y)</math> becomes closely related to the Binomial, giving the identity <math>\Beta(X,Y)</math>{{←→}}<source lang=apl inline>÷Y×(X-1)!X+Y-1</source>.
In multiple implementations where [[Factorial]] is extended to use the [[wikipedia:Gamma function|Gamma function]] <math>\Gamma(n)</math>, Binomial is defined to use the above equality for non-integers. In that case, the [[wikipedia:Beta function|Beta function]] <math>\Beta(x,y)</math> becomes closely related to the Binomial, giving the identity <math>\Beta(X,Y)</math>{{←→}}<syntaxhighlight lang=apl inline>÷Y×(X-1)!X+Y-1</syntaxhighlight>.


<source lang=apl>
<syntaxhighlight lang=apl>
       1 1.2 1.4 1.6 1.8 2!5
       1 1.2 1.4 1.6 1.8 2!5
5 6.105689248 7.219424686 8.281104786 9.227916704 10
5 6.105689248 7.219424686 8.281104786 9.227916704 10
       2!3j2
       2!3j2
1J5
1J5
</source>{{Works in|[[Dyalog APL]]}}
</syntaxhighlight>{{Works in|[[Dyalog APL]]}}


== External links ==
== External links ==

Latest revision as of 21:50, 10 September 2022

!

Binomial (!) is a dyadic scalar function which gives the binomial coefficient between the two arguments. The argument order k!n is reversed compared to most of traditional mathematical notation's alternative notations, for example and , but not others, like . Binomial shares the glyph ! with the monadic arithmetic function Factorial.

Examples

For non-negative integer arguments, the binomial coefficient k!n is equal to the number of ways to choose k items out of n distinct items. For example, 3!5 is 10 because there are 10 ways to pick 3 items out of 5: 123, 124, 125, 134, 135, 145, 234, 235, 245, 345.

      0 1 2 3 4 5!5
1 5 10 10 5 1

k!n also corresponds to the k-th value (zero-indexed) on the n-th row (also zero-indexed) of Pascal's triangle.

      ⍉∘.!⍨ 0,⍳5
1 0  0  0 0 0
1 1  0  0 0 0
1 2  1  0 0 0
1 3  3  1 0 0
1 4  6  4 1 0
1 5 10 10 5 1
Works in: Dyalog APL

Properties

The value of X!Y equals (!Y)÷(!X)×!Y-X.

      Alt←{(!⍵)÷(!⍺)×!⍵-⍺}
      0 1 2 3 4 5 Alt 5
1 5 10 10 5 1

In multiple implementations where Factorial is extended to use the Gamma function , Binomial is defined to use the above equality for non-integers. In that case, the Beta function becomes closely related to the Binomial, giving the identity ÷Y×(X-1)!X+Y-1.

      1 1.2 1.4 1.6 1.8 2!5
5 6.105689248 7.219424686 8.281104786 9.227916704 10
      2!3j2
1J5
Works in: Dyalog APL

External links

Documentation

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