(In reply to
re(2): Reason why sum of limits is not same as limit as sums by np_rt)
"So in summary, the theorem that says that the sum of limits is the same as the limit of the sums does not hold for an infinite number of functions."
The sum, being one function of n, is just one function, so what is meant is and infinite number of terms. However, it does hold for some functions that involve an infinite number of terms, such as
sin(x)= x  x^3/3! + x^5/5!  x^7/7! + ...
Each term smoothly approaches the value it has for a given x, and the total of the limits equals the limit of the total.
The way this differs from the function in the problem is best shown by individual values:
f(3) = 1/4 + 1/5 + 1/6
f(4) = 1/5 + 1/6 + 1/7 + 1/8
f(5) = 1/6 + 1/7 + 1/8 + 1/9 + 1/10
In this instance we can't keep track of "a given term" to find its limit. The number of terms keeps increasing.
The same could conceivably happen in a series that starts out as infinite (the way the Maclaurin series for sine starts out as infinite, but differently defined), so long as there was not a 1to1 correspondence of terms from one iteration of the series to the next. But the problem lies in the lack of correspondence of terms in successive valuationsnot in the series being infinite.

Posted by Charlie
on 20040820 13:48:41 