Faster submatrix selection

Gary

2011 6 21

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9 次查看（30 天）

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Hi,

I've been trying to use mtimesx function for multidimensional multiplication to multiply M1*M2*.... A trial code for this is:

L=1000;
n=10;
a=rand(n,n,L);
for i=1:L
    a(:,:,i)=expm(1i*(a(:,:,i)+a(:,:,i)')/2);
end  
c=a;
while(L~=1)
    L2=floor(L/2)*2; 
    d=c(:,:,end);
    c = mtimesx(c(:,:,1:2:L2-1),c(:,:,2:2:L2));
    if(L2~=L)
        c(:,:,end+1) = d;
    end
    L=size(c,3);
end

It works quite well and gives a speed up when L is large but is slow when the dimension n is large. It seems to be due to finding the odd and even submatrices c(:,:,1:2:L2-1) and c(:,:,2:2:L2). Does anyones know how to speed up this process?

Thanks