mf_matpowersym.hlp

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{smcl}
{* 25mar2005}{...}
{cmd:help mata matpowersym()}
{hline}
{* index matpowersym()}{...}
{* index power}{...}

{title:Title}

{p 4 4 2}
{bf:[M-5] matpowersym() -- Powers of a symmetric matrix}


{title:Syntax}

{p 8 35 2}
{it:numeric matrix}
{cmd:matpowersym(}{it:numeric matrix A}{cmd:,}
{it:real scalar p}{cmd:)}

{p 8 35 2}
{it:void}{bind:         }
{cmd:_matpowersym(}{it:numeric matrix A}{cmd:,}
{it:real scalar p}{cmd:)}


{title:Description}

{p 4 4 2}
{cmd:matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)} 
returns {it:A^p} for symmetric matrix or Hermitian matrix {it:A}.
The matrix returned is real if {it:A} is real, and complex is {it:A} is 
complex.  

{p 4 4 2}
{cmd:_matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)} 
does the same thing, but instead of returning the result, it stores the 
result in {it:A}.


{title:Remarks}

{p 4 4 2}
Do not confuse 
{cmd:matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)} 
and 
{it:A}{cmd::^}{it:p}.  If {it:p}==2, the first returns {it:A}{cmd:*}{it:A} and
the second returns {it:A} with each element squared.  

{p 4 4 2}
Powers can be positive, negative, integer, or noninteger.  Thus
{cmd:matpowersym(}{it:A}{cmd:, .5)} is a way to find the square-root matrix
{it:R} such that {it:R}{cmd:*}{it:R}=={it:A}, and
{cmd:matpowersym(}{it:A}{cmd:, -1)} is a way to find the inverse.  In the case
of inversion, you could obtain the result more quickly using other
routines.

{p 4 4 2}
Powers are obtained by extracting the eigenvalues and eigenvectors of 
{it:A}, raising the eigenvalues to the specified power, and then rebuilding 
the matrix.  That is, first {it:X} and {it:L} are found such that 

			{it:A}{it:X} = {it:X}*diag({it:L}){right:(1)   }

{p 4 4 2}
In the case of symmetric (Hermitian) matrix {it:A}, {it:X} is orthogonal, meaning 
{it:X}'{it:X} = {it:X}{it:X}' = {it:I}.  Thus

			{it:A} = {it:X}*diag({it:L})*{it:X}'{right:(2)   }

{p 4 4 2}
{it:A}^{it:p} is then defined 

			{it:A} = {it:X}*diag({it:L}):^{it:p})*{it:X}'{right:(3)   }

{p 4 4 2}
(1) is obtained via {cmd:symeigensystem()}; see 
{bf:{help mf_eigensystem:[M-5] eigensystem()}}.



{title:Conformability}

    {cmd:matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)}:
		{it:A}:  {it:n x n}
		{it:p}:  1 {it:x} 1
	   {it:result}:  {it:n x n}

    {cmd:_matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)}:
	{it:input:}
		{it:A}:  {it:n x n}
		{it:p}:  1 {it:x} 1
	{it:output:}
		{it:A}:  {it:n x n}

		
{title:Diagnostics}

{p 4 4 2}
{cmd:matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)}
and 
{cmd:_matpowersym(}{it:A}{cmd:,} {it:p}{cmd:)}
return missing results if {it:A} contains missing values.

{p 4 4 2}
In addition:

{p 8 12 2}
1.
These functions do not check that {it:A} is symmetric or Hermitian. If {it:A}
is a real matrix, only the lower triangle, including the diagonal, is used.
If {it:A} is a complex matrix, only the lower triangle and the real parts of
the diagonal elements are used.

{p 8 12 2}
2.  
These functions return a matrix of the same storage type as {it:A}.  
That means that if {it:A} is real and {it:A}^{it:p} cannot be expressed 
as a real, a matrix of missing values is returned.  If you want the 
generalized solution, code {cmd:matpowersym(C(}{it:A}{cmd:),} {it:p}{cmd:)}.
This is the same rule as with scalars:  (-1){cmd:^}.5 is missing, but 
{cmd:C(}-1{cmd:)^}.5 is 1i.

{p 8 12 2}
3.  
These functions are guaranteed to return a matrix that is numerically
symmetric, Hermitian, or symmetriconly if theory states that the matrix should
be symmetric, Hermitian, or symmetriconly.

{p 12 12 2}
Concerning theory, the returned result is not necessarily symmetric
(Hermitian).  The eigenvalues {it:L} of a symmetric (Hermitian) matrix are 
real.  If {it:L}{cmd::^}{it:p} are real, then the returned matrix will be 
symmetric (Hermitian), but otherwise, it will not.  Think of a negative
eigenvalue and {it:p}=.5:  this results in a complex eigenvalue for
{it:A}^{it:p}.  In that case, if the original matrix was real (the
eigenvectors were real), then the resulting matrix will be symmetriconly.  If
the original matrix was complex (the eigenvectors were complex), then the
resulting matrix will have no special structure.


{title:Source code}

{p 4 4 2}
{view matpowersym.mata, adopath asis:matpowersym.mata},
{view _matpowersym.mata, adopath asis:_matpowersym.mata},
{view _symmatfunc_work.mata, adopath asis:_symmatfunc_work.mata}

    
{title:Also see}

{p 4 13 2}
Manual:  {hi:[M-5] matpowersym()}

{p 4 13 2}
Online:  help for 
{bf:{help mf_matexpsym:[M-5] matexpsym()}},
{bf:{help mf_eigensystem:[M-5] eigensystem()}};
{bf:{help m4_matrix:[M-4] matrix}}
{p_end}