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MathGroup Archive 1992

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Determinant Function (Correction & Speedup)

  • To: mathgroup <mathgroup at yoda.physics.unc.edu>
  • Subject: Determinant Function (Correction & Speedup)
  • From: HAY at leicester.ac.uk
  • Date: Wed, 22 APR 92 09:43:15 GMT

Correction: multiply the output of NewDet3 by Signature[M] to take care of the
case when this is -1, eg  {{2,1}, {1,2}}.

Speedup: use Transpose once instead of twice - 
getting to know Transpose better has been one of the benefits of keeping at this
problem.

NewDet4[M_] :=
	( Signature[M]    
			Plus@@Times@@
			Append[ Transpose[#,{2,1,1}],(Signature/@#) ]&@
			Permutations[ M ]
	)
 

NewDet4[{{2,1},{1,2}}]
	3


The timings are (on MacIIfx Mma Version 2.03)

a[n_] := Table[ StringJoin["a",ToString[i],ToString[j]], {i,1,n},{j,1,n}]

Do[ Print[ First[ Timing[ NewDet4[ a[i]]]]], {i,1,7}]

	0.0666667 Second
	0.0666667 Second
	0.116667 Second
	0.233333 Second
	0.716667 Second
	3.73333 Second
	28.0167 Second
	
I am grateful for two suggestions  for speeding up NewDet3:
	
FRANK ZIZZA <zizza at edu.willamette.pioneer>

suggests getting rid of the second Transpose by using Thread.
 
This seems to work for the smaller sizes but it is slower than NewDet3 for 7 by
7 determinants,at least on my machine:

NewDetZizza[M_] :=
	(	Signature[M]
   Plus@@Times@@
				Append[ Thread[Transpose[#,{1,2,2}]],Signature/@#]&@
    Permutations[M]
 )
        
Do[ Print[ First[ Timing[ NewDetZizza[ a[i]]]]], {i,1,7}]

0.0666667 Second
0.0666667 Second
0.133333 Second
0.266667 Second
0.75 Second
4. Second
35.5167 Second

The timings for New Det3 were

Timings[NewDet3]
0.25 Second
0.0666667 Second
0.116667 Second
0.25 Second
0.75 Second
4.03333 Second
31.0167 Second

DAVID SIBLEY <sibley at edu.psu.math> says

>"Permutations" always seems to return a list whose signatures are in the
>order 1,-1,-1,1,1,-1,-1,1,1,... There must be some way to take advantage
>of this, something better than actually computing the signatures."

but this pattern breaks down on Range[5] at the 25 th entry

{1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1,

  -1, [[ here 1 ]], -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1,
1, 1, -1, 
  -1, 1, 1, -1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1,

  -1, 1, 1, -1, -1, 1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1,

  -1, 1, 1, -1, -1, 1, 1, -1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, -1,

  -1, 1, 1, -1, -1, 1, 1, -1, -1, 1}
  
Any suggestions on this?


From
Allan Hayes
Department of Mathematics
The University
Leicester LE1 7RH
U.K.
hay at leicester.ac.uk





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