Sunday, 28 July 2013

real analysis - Summation Symbol: Changing the Order




I have some questions regarding the order of the summation signs (I have tried things out and also read the wikipedia page, nevertheless some questions remained unanswered):



Original 1. wikipedia says that:



mk=1aknk=1bl=mk=1nl=1akbl



does not necessarily hold. What would be a concrete example for that?



Edited 1. wikipedia says that:




mk=1aknl=1bl=mk=1nl=1akbl



does not necessarily hold. What would be a concrete example for that?



2.As far as I see generally it holds that:



mj=1ni=1aibj=ni=1mj=1aibj



why is that? It is not due to the property, that multiplication is commutative, is it?




3.What about infinite series, when does:
k=1l=1akbl=k=1akl=1bl hold?
And does here too k=1l=1akbl=l=1k=1akbl hold?



Thanks


Answer



For the *original first question where l=k, let m=n=2, a1=b1=1, and a2=b2=2; then



2k=1ak2k=1bk=2k=1ak(1+2)=13+23=9,




but 2k=12k=1akbk=2k=1(12+22)=5+5=10.



For the second question, imagine arranging the terms aibj in an n×m array:



a1b1a1b2a1b3a1bmmj=1a1bja2b1a2b2a2b3a2bmmj=1a2bja3b1a3b2a3b3a3bmmj=1a3bjanb1anb2anb3anbmmj=1anbjni=1aib1ni=1aib2ni=1aib3ni=1aibm



For each j=1,,m, ni=1aibj is the sum of the entries in column j, and for each i=1,,n, mj=1aibj is the sum of the entries in row i. Thus,



mj=1ni=1aibj=mj=1sum of column j=ni=1sum of row i=ni=1mj=1aibj.




For infinite double series the situation is a bit more complicated, since an infinite series need not converge. However, it is at least true that if either of



mj=1ni=1|aibj|andni=1mj=1|aibj|



converges, then the series without the absolute values converge and are equal. This PDF has much more information on double sequences and series.


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