Again, that’s assuming the conclusion; what if 1 − 0.999… weren’t zero, and you picked that as epsilon? You’re skipping steps. It’s worth writing down exactly what you think is happening more carefully.
(To be clear, I’m not claiming that you’ve asserted any false statements, but I think there’s an important sense in which you aren’t taking seriously the hypothetical world in which 1 − 0.999… isn’t zero, and what that world might look like. There’s something to learn from doing this, I think.)
Alice: 1 = 0.999... Bob: No, they’re different. Alice: Okay, if they’re different then why do you get zero if you subtract one from the other? Bob: You don’t, you get 0.000...0001. Alice: How many zeros are there? Bob: An infinite number of them. Then after the last zero, there’s a one.
Alice is right (as far as real numbers go) but at this point in the discussion
she has not yet proved her case; she needs to argue to Bob that he shouldn’t
use the concept “the last thing in an infinite sequence” (or that if he does
use it he needs to define it more rigorously).
you aren’t taking seriously the hypothetical world in which 1 − 0.999… isn’t zero
In this (math) world it is zero only because for every nonzero positive epsilon, you can pick a FINITE number of 9s, such that 1-0.999999...99999 (a FINITE number of 9s) is already SMALLER than that epsilon.
For EVERY real number greater than zero, you have a FINITE number of 9s, such that this difference is smaller.
Therefore the difference cannot by a number greater then 0.
Again, that’s assuming the conclusion; what if 1 − 0.999… weren’t zero, and you picked that as epsilon? You’re skipping steps. It’s worth writing down exactly what you think is happening more carefully.
(To be clear, I’m not claiming that you’ve asserted any false statements, but I think there’s an important sense in which you aren’t taking seriously the hypothetical world in which 1 − 0.999… isn’t zero, and what that world might look like. There’s something to learn from doing this, I think.)
If I may, let me agree with you in dialogue form:
Alice: 1 = 0.999...
Bob: No, they’re different.
Alice: Okay, if they’re different then why do you get zero if you subtract one from the other?
Bob: You don’t, you get 0.000...0001.
Alice: How many zeros are there?
Bob: An infinite number of them. Then after the last zero, there’s a one.
Alice is right (as far as real numbers go) but at this point in the discussion she has not yet proved her case; she needs to argue to Bob that he shouldn’t use the concept “the last thing in an infinite sequence” (or that if he does use it he needs to define it more rigorously).
There is no “after the last” zero.
In this (math) world it is zero only because for every nonzero positive epsilon, you can pick a FINITE number of 9s, such that 1-0.999999...99999 (a FINITE number of 9s) is already SMALLER than that epsilon.
For EVERY real number greater than zero, you have a FINITE number of 9s, such that this difference is smaller.
Therefore the difference cannot by a number greater then 0.