2,259 reputation
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bio website math.cornell.edu/~justin
location Ithaca, NY
age 40
visits member for 3 years, 11 months
seen Aug 24 at 19:40
I'm a set theorist working at Cornell. Most of my work has been with forcing axioms and the combinatorics and Ramsey theory of the first uncountable cardinal. I am also interested in ways in which set theory can be applied to other areas of mathematics. Additionally, I have interests in group theory (amenability in particular).

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awarded  Curious
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accepted What is the effect of adding 1/2 to a continued fraction?
May
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comment What is the effect of adding 1/2 to a continued fraction?
Noam, thanks. I've been away from MO for a while, but this was actually exactly what I was looking for.
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awarded  Popular Question
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awarded  Good Question
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awarded  Popular Question
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awarded  Nice Answer
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awarded  Revival
Apr
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comment What is the effect of adding 1/2 to a continued fraction?
@Douglas: Could you contact me off list?
Apr
30
comment What is the effect of adding 1/2 to a continued fraction?
@Douglas: Actually you might be right. The formula in my comment is a little cleaner though.
Apr
30
comment What is the effect of adding 1/2 to a continued fraction?
@Douglas: The cases depending on whether a1 > 1 seems off. I think the (or rather a) formula is 1/2 x = [a0/2,2x'] or [(a0-1)/2,1,1,(x'-1)/2] (depending on parity of a0) and 2x=[2a0,x'/2]. This gives x/2 = [a0/2,2a1,x''/2] or [(a0-1)/2,1,1,(x'-1)/2]. Thanks again; this was exactly what I was looking for. I had dismissed finding a rule this simple for some reason.
Apr
29
comment What is the effect of adding 1/2 to a continued fraction?
@Douglas: Check your formula for doubling. I think it is not quite correct.
Apr
26
comment What is the effect of adding 1/2 to a continued fraction?
@Douglas: thanks, but this is not really what I'm asking. The 2[a0;2a1,a2,2a3,a4,...]=[2a0;a1,2a2,a3,2a4,...] part is in the spirit of what I want, but I want something analogous which works for any sequence. I was aware of this observation --- the hard part is of course in dealing with the remainders.
Apr
26
comment What is the effect of adding 1/2 to a continued fraction?
Thanks, but this is not really what I'm asking. See the comments in the edit portion of the question.
Apr
26
revised What is the effect of adding 1/2 to a continued fraction?
added the "edit:" part of the text
Apr
26
comment What is the effect of adding 1/2 to a continued fraction?
@David:Actually I think this is not quite true. Unless I am missing something, this is not reversible. You can, however, multiply by 4 by adding 1/2 and reciprocating though. The point is that multiplication by 1/2 corresponds to operations in PSL_2(Z[1/sqrt(2)]), while adding 1/2 comes from PSL_2(Z[1/2]).