A question about an infinite product

In summary, the conversation discusses an infinite product that results in a negative value even though all of the terms in the product are positive. The conversation suggests taking a limit to resolve this issue, but it is pointed out that the limit does not exist. Thus, a valid expression for A-B cannot be written as an infinite product.
  • #1
WACG
7
0
I am puzzled by the following infinite product:
Let B > A

A - B = [A^(1/2) + B^(1/2)] * [A^(1/2) - B^(1/2)]
= [A^(1/2) + B^(1/2)] * [A^(1/4) + B^(1/4)] * [A^(1/4) - B^(1/4)]

=[A^(1/2) + B^(1/2)] * [A^(1/4) + B^(1/4)] * [A^(1/8) + B^(1/8)] * [A^(1/8) - B^(1/8)]

etc.

Continuing the obvious expansion into an infinite product produces a sequence of terms none of which are negative. However, since B > A then A - B is a negative value. How can a infinite product of terms greater than zero produce a negative value? Surely there is a "simple" explanation.

Thanks for any comments.
 
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  • #2
WACG said:
I am puzzled by the following infinite product:
Let B > A

A - B = [A^(1/2) + B^(1/2)] * [A^(1/2) - B^(1/2)]
= [A^(1/2) + B^(1/2)] * [A^(1/4) + B^(1/4)] * [A^(1/4) - B^(1/4)]

=[A^(1/2) + B^(1/2)] * [A^(1/4) + B^(1/4)] * [A^(1/8) + B^(1/8)] * [A^(1/8) - B^(1/8)]

etc.

Continuing the obvious expansion into an infinite product produces a sequence of terms none of which are negative. However, since B > A then A - B is a negative value. How can a infinite product of terms greater than zero produce a negative value? Surely there is a "simple" explanation.

Thanks for any comments.
In each of the products the last term to the right is negative.
 
  • #3
well there's that last term [A^(1/(2^n) - B^(1/(2^n)], you can't ignore it,
unless you prove that its limit as n->infinity is equal to 1, and it isn't, because it's always negative.
 
  • #4
Hi, WACG,
you are proposing that[tex]A-B = \lim_{n \to \infty} \prod_{i=1}^n (A^{1/2^i} + B^{1/2^i})[/tex]when in fact this limit does not exist: the factor [itex]A^{1/2^i} + B^{1/2^i}[/itex] tends to 1+1 = 2 on infinity.

Each of the successive expressions in your original post is made of a larger and larger number (the product of the positives) times a smaller and smaller negative number.

The guys at the Calculus & Analysis forum would probably have something better to say, though.
 
  • #5
Well, I guess my question becomes - How DO you write the indicated infinite product that resolves itself to A-B? In other words, how do you write the product that has the persistent and ever smaller negative number at the tail end?
 
  • #6
I suppose I'd say that, for all integers n > 0,[tex]A-B = \left( \prod_{i=1}^n (A^{1/2^i} + B^{1/2^i}) \right) \cdot (A^{1/2^n} - B^{1/2^n})[/tex]which is pretty much what you wrote -- before trying to produce a limit. The statement is true for all given n, but no limit needs to be involved, as I see it.

For another example, all of the products 1.(1/1), 2.(1/2), 4.(1/4), 8.(1/8), ... equal 1; the fraction is getting smaller and smaller, but that doesn't mean that the sequence 1,2,4,8,... converges, let alone to 1.
 
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  • #7
So you are telling me that a valid expression for A - B cannot be written as an infinite product (regardless whether it needs to be "involved").
 
  • #8
Well, if by an "infinite product" you mean a limit, then no, since the limit does not exist. Presently I can't see any other way of moving from a finite product to an infinite one than by taking a limit, but mind openers are welcome.

P.S.: Obviously, the expression for A-B in post#6 *has* a limit when n->inf, since it has a constant value regardless of n. But if you want to get rid of the difference at the end and use only the product of the positive sums, that one has no limit.

P.P.S: In yet other words, the limit of a product of sequences is the product of their limits, provided these limits exist; in this case, one of them does not.
 
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1. What is an infinite product?

An infinite product is a mathematical concept that represents the multiplication of an infinite number of terms. It is similar to an infinite sum, but instead of adding terms, it multiplies them.

2. How does an infinite product differ from a finite product?

An infinite product differs from a finite product in that it involves multiplying an infinite number of terms, whereas a finite product involves multiplying a finite number of terms.

3. Can an infinite product ever equal zero?

Yes, it is possible for an infinite product to equal zero. This occurs when at least one of the terms being multiplied is equal to zero.

4. What is the value of an infinite product if all the terms are positive?

If all the terms in an infinite product are positive, then the value of the product will either be a positive real number or infinity, depending on the convergence of the product.

5. How is the convergence of an infinite product determined?

The convergence of an infinite product is determined by taking the limit as the number of terms approaches infinity. If this limit exists and is a finite number, then the product is said to converge. If the limit does not exist or is infinite, then the product is said to diverge.

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