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A219507
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Pierce expansion of (5 - sqrt(21))/2.
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2
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4, 6, 109, 111, 1330669, 1330671, 2356194280407770989, 2356194280407770991, 13080769480548649962914459850235688797656360638877986029, 13080769480548649962914459850235688797656360638877986031
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OFFSET
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0,1
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COMMENTS
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For x in the open interval (0,1) define the map f(x) = 1 - x*floor(1/x). The n-th term (n >= 0) in the Pierce expansion of x is given by floor(1/f^(n)(x)), where f^(n)(x) denotes the n-th iterate of the map f, with the convention that f^(0)(x) = x.
The present sequence is the case x = 1/2*(5 - sqrt(21)).
Jeffrey Shallit has shown that the Pierce expansion of the quadratic irrational (c - sqrt(c^2 - 4))/2 has the form [c(0) - 1, c(0) + 1, c(1) - 1, c(1) + 1, c(2) - 1, c(2) + 1, ...], where c(0) = c and c(n+1) = c(n)^3 - 3*c(n). This is the case c = 5. For other cases see A006276 (c = 3), A219506 (c = 4) and A006275 (essentially c = 6 apart from the initial term).
The Pierce expansion of {(c - sqrt(c^2 - 4))/2}^(3^n) is [[c(n) - 1, c(n) + 1, c(n+1) - 1, c(n+1) + 1, c(n+2) - 1,c(n+2) + 1, ...].
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LINKS
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FORMULA
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a(2*n) = (1/2*(5 + sqrt(21)))^(3^n) + (1/2*(5 - sqrt(21)))^(3^n) - 1.
a(2*n+1) = (1/2*(5 + sqrt(21)))^(3^n) + (1/2*(5 - sqrt(21)))^(3^n) + 1.
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EXAMPLE
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Let x = 1/2*(5 - sqrt(21)). We have the alternating series expansions
x = 1/4 - 1/(4*6) + 1/(4*6*109) - 1/(4*6*109*111) + ...
x^3 = 1/109 - 1/(109*111) + 1/(109*111*1330669) - ...
x^9 = 1/1330669 - 1/(1330669*1330671) + ....
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MATHEMATICA
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PierceExp[A_, n_] := Join[Array[1 &, Floor[A]], First@Transpose@ NestList[{Floor[1/Expand[1 - #[[1]] #[[2]]]], Expand[1 - #[[1]] #[[2]]]} &, {Floor[1/(A - Floor[A])], A - Floor[A]}, n - 1]]; PierceExp[N[(5 - Sqrt[21])/2 , 7!], 10] (* G. C. Greubel, Nov 14 2016 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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