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:
m∑k=1akn∑k=1bl=m∑k=1n∑l=1akbl
does not necessarily hold. What would be a concrete example for that?
Edited 1. wikipedia says that:
m∑k=1akn∑l=1bl=m∑k=1n∑l=1akbl
does not necessarily hold. What would be a concrete example for that?
2.As far as I see generally it holds that:
m∑j=1n∑i=1aibj=n∑i=1m∑j=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=1∞∑l=1akbl=∞∑k=1ak∞∑l=1bl hold?
And does here too ∞∑k=1∞∑l=1akbl=∞∑l=1∞∑k=1akbl hold?
Thanks
Answer
For the *original first question where l=k, let m=n=2, a1=b1=1, and a2=b2=2; then
2∑k=1ak2∑k=1bk=2∑k=1ak(1+2)=1⋅3+2⋅3=9,
but 2∑k=12∑k=1akbk=2∑k=1(12+22)=5+5=10.
For the second question, imagine arranging the terms aibj in an n×m array:
a1b1a1b2a1b3…a1bm∑mj=1a1bja2b1a2b2a2b3…a2bm∑mj=1a2bja3b1a3b2a3b3…a3bm∑mj=1a3bj⋮⋮⋮⋮⋮anb1anb2anb3…anbm∑mj=1anbj∑ni=1aib1∑ni=1aib2∑ni=1aib3…∑ni=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,
m∑j=1n∑i=1aibj=m∑j=1sum of column j=n∑i=1sum of row i=n∑i=1m∑j=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
m∑j=1n∑i=1|aibj|andn∑i=1m∑j=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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