A curious approximation to the Feigenbaum constant is given by
(1)
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where is Gelfond's constant, which is good to 6 digits to the right of the decimal point.
M. Trott (pers. comm., May 6, 2008) noted
(2)
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where is Gauss's constant, which is good to 4 decimal digits, and
(3)
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where is the tetranacci constant, which is good to 3 decimal digits.
A strange approximation good to five digits is given by the solution to
(4)
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which is
(5)
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where is the Lambert W-function (G. Deppe, pers. comm., Feb. 27, 2003).
(6)
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gives to 3 digits (S. Plouffe, pers. comm., Apr. 10, 2006).
M. Hudson (pers. comm., Nov. 20, 2004) gave
(7)
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(8)
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(9)
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which are good to 17, 13, and 9 digits respectively.
Stoschek gave the strange approximation
(10)
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which is good to 9 digits.
R. Phillips (pers. comm., Sept. 14, 2004-Jan. 25, 2005) gave the approximations
(11)
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(12)
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(13)
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(14)
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(15)
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(16)
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where e is the base of the natural logarithm and is Gelfond's constant, which are good to 3, 3, 5, 7, 9, and 10 decimal digits, respectively, and
(17)
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(18)
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(19)
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(20)
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(21)
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(22)
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(23)
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which are good to 3, 3, 3, 4, 6, 8, and 8 decimal digits, respectively.
An approximation to due to R. Phillips (pers. comm., Jan. 27, 2005) is obtained by numerically solving
(24)
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for , where is the golden ratio, which is good to 4 digits.