'\"
'\" Generated from file 'combinatorics\&.man' by tcllib/doctools with format 'nroff'
'\"
.TH "math::combinatorics" 3tcl 1\&.2\&.3 tcllib "Tcl Math Library"
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.SH NAME
math::combinatorics \- Combinatorial functions in the Tcl Math Library
.SH SYNOPSIS
package require \fBTcl  8\&.2\fR
.sp
package require \fBmath  ?1\&.2\&.3?\fR
.sp
\fB::math::ln_Gamma\fR \fIz\fR
.sp
\fB::math::factorial\fR \fIx\fR
.sp
\fB::math::choose\fR \fIn k\fR
.sp
\fB::math::Beta\fR \fIz w\fR
.sp
.BE
.SH DESCRIPTION
.PP
The \fBmath\fR package contains implementations of several
functions useful in combinatorial problems\&.
.SH COMMANDS
.TP
\fB::math::ln_Gamma\fR \fIz\fR
Returns the natural logarithm of the Gamma function for the argument
\fIz\fR\&.
.sp
The Gamma function is defined as the improper integral from zero to
positive infinity of
.CS


  t**(x-1)*exp(-t) dt

.CE
.sp
The approximation used in the Tcl Math Library is from Lanczos,
\fIISIAM J\&. Numerical Analysis, series B,\fR volume 1, p\&. 86\&.
For "\fBx\fR > 1", the absolute error of the result is claimed to be
smaller than 5\&.5*10**-10 -- that is, the resulting value of Gamma when
.CS


  exp( ln_Gamma( x) )

.CE
.IP
is computed is expected to be precise to better than nine significant
figures\&.
.TP
\fB::math::factorial\fR \fIx\fR
Returns the factorial of the argument \fIx\fR\&.
.sp
For integer \fIx\fR, 0 <= \fIx\fR <= 12, an exact integer result is
returned\&.
.sp
For integer \fIx\fR, 13 <= \fIx\fR <= 21, an exact floating-point
result is returned on machines with IEEE floating point\&.
.sp
For integer \fIx\fR, 22 <= \fIx\fR <= 170, the result is exact to 1
ULP\&.
.sp
For real \fIx\fR, \fIx\fR >= 0, the result is approximated by
computing \fIGamma(x+1)\fR using the \fB::math::ln_Gamma\fR
function, and the result is expected to be precise to better than nine
significant figures\&.
.sp
It is an error to present \fIx\fR <= -1 or \fIx\fR > 170, or a value
of \fIx\fR that is not numeric\&.
.TP
\fB::math::choose\fR \fIn k\fR
Returns the binomial coefficient \fIC(n, k)\fR
.CS


   C(n,k) = n! / k! (n-k)!

.CE
.IP
If both parameters are integers and the result fits in 32 bits, the
result is rounded to an integer\&.
.sp
Integer results are exact up to at least \fIn\fR = 34\&.  Floating point
results are precise to better than nine significant figures\&.
.TP
\fB::math::Beta\fR \fIz w\fR
Returns the Beta function of the parameters \fIz\fR and \fIw\fR\&.
.CS


   Beta(z,w) = Beta(w,z) = Gamma(z) * Gamma(w) / Gamma(z+w)

.CE
.IP
Results are returned as a floating point number precise to better than
nine significant digits provided that \fIw\fR and \fIz\fR are both at
least 1\&.
.PP
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fImath\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH CATEGORY
Mathematics