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- July 18th 2010, 02:33 PM #1

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## Help with a problem

Simple problem from Amazon.com: Discovering Modern Set Theory. I: The Basics (Graduate Studies in Mathematics, Vol…

We define a function G from [0, 1) x [0, 1) to [0, 1) as follows:

For any <x, y> in [0, 1) x [0, 1) such that x = 0.x1x2x3x4x5... and y = y1y2y3y4y5... G(x, y) = 0.x1y1x2y2x3y3....

Clearly this is into. However, in the book they claim it is not onto. I can't see why this is. My reasoning is:

Take any z in [0, 1) such that z = 0.z1z2z3z4z5.... then x = 0.z1z3z5z7... and y = 0.z2z4z6z8... will be such that G(x, y) = z.

Any help would be great.

Regards

Sam

- July 18th 2010, 02:46 PM #2

- July 18th 2010, 02:54 PM #3

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Hi!

Yea, sorry about that; I'm Latex intolerant!

The problem is also here on page 35 Discovering Modern Set Theory: The ... - Google Books

Thanks

Sam

- July 18th 2010, 03:03 PM #4

- July 18th 2010, 03:07 PM #5

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- July 18th 2010, 03:34 PM #6
You're welcome! But after some thought, I have reservations about whether the function is actually a function.

Consider (x,y) = (0.19999..., 0.19999...) = (0.20000..., 0.20000...).

Applying G to the first yields G(x,y) = 0.119999... = 0.12

Applying G to the second yields G(x,y) = 0.220000... = 0.22

Also, G(0.19999..., 0.20000...) = 0.12909090... which is again different.

Thoughts?

EDIT: Oh I see this is mentioned at the transition between page 34 and page 35. So the function is a valid function, and the thing about z = 0.90909090... still holds.

- July 19th 2010, 12:13 AM #7

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