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LostInParadise's avatar

Can you find a use for this?

Asked by LostInParadise (31916points) April 3rd, 2009

Or what I did when I could not get back to sleep.

If you take two numbers x and y, two obvious things to do with them are to add and subtract them giving x+y and x-y. If the original numbers were far apart then the new pair will be close together and vice versa. For example if x=10 and y=1 then you get 11 and 9. Adding and subtracting these two numbers gives 20 and 2, double the starting values, which will be the general case and means that the numbers have the same ratio as the starting pair.

This got me to wondering if there was a ratio of x to y which would give the same ratio for x-y and x+y. If we take x=1 and y=r then we get r/1 = (1-r)/(1+r). This gives r=sqrt(2) – 1, which I dub LostInParadise’s constant.

It seems to me that there should be some signficance to the constant, though I can’t think of what it could be. Have you got any ideas?

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7 Answers

JellyB's avatar

Eh?

This sounds like a question for Mattbrowne…. :)

mattbrowne's avatar

Your constant might have a relevance in trigonometry, because

sqrt(2)–1 = tan(pi/8)

majamin's avatar

It smells vaguely of arithmetic/geometric means… and the beginning of some kind of iteration… also, you might want to ask… is the second root r = – sqrt(2) – 1 special in any way?

LostInParadise's avatar

There is a way of describing this in terms of matrices and eigenvalues, which is really dull even if you know what I am talking about. I hope there is something better.

mattbrowne's avatar

@LostInParadise – It was 25 years ago when I last dealt with the eigendecomposition of a matrix during my computer science studies. I have advanced math knowledge, but I’m not an expert. Maybe someone on Fluther is.

BonusQuestion's avatar

This constant is the inverse of silver ratio.

Its continued fraction has a nice format. It is in the form: 1/( 2+ 1/(2+ 1/(2+ 1/(2+ ....) ) ) )

This continued fraction is denoted sometimes by [0, 2, 2, 2, 2, ...].

LostInParadise's avatar

Fantastic! So I am not crazy, well at least not for thinking the number has some significance.

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