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gwsinger
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There are rationals between any two reals? -- Rudin 1.20
In Rudin 1.20(b), Rudin proves that between any two real numbers x and y, there exists a rational number p. His strategy is to "blow up" x and y by some integer n (generated by the archimedian property) such that ny - nx > 1. Then he attempts to locate another integer m that is between nx and ny, such that m - 1 < nx < m < ny. From here we have
nx < m < ny
and when we multiply each term by (1/n) we can say that
x < m/n < y where p = m/n. Hence the proof is done.
Now my question is as follows: how can Rudin just pick an m such that m - 1 < nx < m < ny?
I certainly understand that we can use the archimedian property to find an m greater than nx, but how can we be SURE that this m is less than ny and even less than 1 unit distance greater than nx?
Here is the proof in full from the text in case it helps:
In Rudin 1.20(b), Rudin proves that between any two real numbers x and y, there exists a rational number p. His strategy is to "blow up" x and y by some integer n (generated by the archimedian property) such that ny - nx > 1. Then he attempts to locate another integer m that is between nx and ny, such that m - 1 < nx < m < ny. From here we have
nx < m < ny
and when we multiply each term by (1/n) we can say that
x < m/n < y where p = m/n. Hence the proof is done.
Now my question is as follows: how can Rudin just pick an m such that m - 1 < nx < m < ny?
I certainly understand that we can use the archimedian property to find an m greater than nx, but how can we be SURE that this m is less than ny and even less than 1 unit distance greater than nx?
Here is the proof in full from the text in case it helps:
Pg 9
If x in R, y in R, and x<y, then there exists a p in Q such that x<p<y.
Proof:
Since x<y, we have y-x>0, and the archimedean property furnishes a positive integer n such that
n(y-x)>1.
Apply the archimedean property again, to obtain positive integers m1 and m2 such that m1>nx, m2>-nx. Then
-m2<nx<m1.
Hence there is an integer m ( with -m2<= m <= m1) such that
m-1 <= nx < m
If we combine these inequalities, we obtain
nx<m<= 1 + nx < ny.
Since n>0, it follows that
x < m/n < y. This proves (b) with p = m/n.