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# Talk:Necessary and sufficient

From Citizendium

## Circle

"A circle (in the plane) is usually defined as a curve such that its points all have the same distance from a given point" — a bit problematic; say, half a circle, is it also a circle? It IS "a curve such that its points all have the same distance from a given point", isn't it? Boris Tsirelson 10:05, 2 February 2010 (UTC)

- True, of course. I wanted to keep this colloquial, and there this distinction is usually not made. (It holds for both characterizations.) And since I did not think of the formulation I found now ...
- By the way: Boris, do you think (agree) that this topic is worth a page (for non-mathematicians) like this?
- --Peter Schmitt 10:36, 2 February 2010 (UTC)

- It was a bit unexpected for me. However, why not? It could help. And you are not the first to try such things (but probably one of the first ten). Look at site of Timothy Gowers, especially, "Miscellaneous" (near the end). Boris Tsirelson 16:12, 2 February 2010 (UTC)