02 Jul '16 20:12>
Originally posted by humyI am not sure what you mean. What about 1.1?
doesn't work so there is no smallest real that works for that.
Originally posted by twhiteheadWhat about it?
I am not sure what you mean. What about 1.1?
Originally posted by humyOK, I wasn't sure what you meant but now I see.
What about it?
1.1 is not the smallest number larger than 1, which is what the 'smallest' would mean in this very narrow context of the constant n that is required in my formula for it to work.
That is because there is no smallest number larger than 1.
Originally posted by twhiteheadYou have just giving me an idea how to modify that to make it work for a greater range of functions including some oscillating ones:
Just for fun, because you know that f(x)<L and f(y)<L you can drop the absolutes and rearrange to get:
lim {x→∞} f(x) = L
⇒
∀x ∈ ℝ : ∃y ∈ ℝ : y>x ∧ f(y)> (L+f(x))/2
Originally posted by humyI don't understand what y is doing, especially in the second line where it is >x and <z but otherwise appears to serve no purpose.
lim {x→∞} f(x) = L ∧ ∀y ∈ ℝ : f(y) < L
⇒
∀x ∈ ℝ : ∃y ∈ ℝ : y>x ∧ ∀z ∈ ℝz>y : f(z) > (L+f(x))/2
" (L+f(x))/2 " can be interpreted as simply meaning "the average of L and f(x)".
Originally posted by twhiteheadOh yes. So, improvement:
I don't understand what y is doing, especially in the second line where it is >x and <z but otherwise appears to serve no purpose.
In the first line I believe you could use x again without issue.
Originally posted by twhiteheadSo it should be:
Actually, my bad. I think the x,y and z were necessary in the second line. I don't think the y in the first was necessary. Certainly, as it stands it doesn't make sense.
Originally posted by humyMiner point, but; Just noticed that it would be more consistent with conventional notation to write that subscript as "z>y" as just ">y" thus the formula should be;
So it should be:
lim {x→∞} f(x) = L ∧ ∀x ∈ ℝ : f(x) < L
⇒
∀x ∈ ℝ : ∃y ∈ ℝ : y>x ∧ ∀z ∈ ℝ{z>y} : f(z) > (L+f(x))/2
also just noticed I also made the error of forgetting to indicate with curly brackets that "z>y" is a subscript last time. Having it written as " ℝz>y " there is bad notation.
Originally posted by humyIt depends on what you mean by the symbol ∞.
If we made x = ∞, would the expression of, say;
x < 2*x
be 'false'?
Or would that be neither 'true' or 'false' but rather 'meaningless' as in 'nonsense'?
Originally posted by twhiteheadperhaps a better question is does either
It depends on what you mean by the symbol ∞.
If you mean 'the Real number infinity', then you are making an error in the first expression as infinity isn't a number. For other meanings / definitions of the symbol, it will depend on the definition as to whether the initial expression makes sense and whether or not the later expressions may be evaluated.