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Not Enough Information about ... Ellipses

Date: 12/01/2010 at 13:17:46
From: Jane
Subject: Would you consider 3.666.... a rational number?

Would you consider this a rational number even though you do not really
know what the next digit is?


It could be 3.666789....

I would say that if the digit repeats, it should have a line over the last

Thank you

Date: 12/01/2010 at 13:40:00
From: Doctor Peterson
Subject: Re: Would you consider 3.666.... a rational number?

Hi, Jane.

If I saw this number on a calculator that showed only the digits 3.666 and
implied that there are more digits after it, I would have no idea what the
number actually is, or whether it is rational.

The ellipsis "..." is used in two slightly different ways. Sometimes it
just means "there's more here that I left out" (as in the calculator
example). Other times it means "continue the pattern you see." The latter
is a little ambiguous, since you really don't know what the pattern is;
but it's generally used only in contexts where it is understood what kind
of pattern to expect. One of those is to indicate a repeating decimal.

So you're right that, in general, you can't tell whether 3.666... is
rational or not; but in some contexts it would be somewhat of a quibble to
insist that it can't be understood as a repeating decimal, even though the
explicit notation is a far better way to say it.

I suspect that often the ellipsis is used for a repeating decimal just
because (a) the bar notation may not be understood by the reader, or (b)
it is too hard to typeset the bar, so they use the next best thing. I like
the notation 3.(6), which overcomes the second issue, though few people
would understand it.

- Doctor Peterson, The Math Forum 
Associated Topics:
Middle School Number Sense/About Numbers

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