An amazing pandigital approximation to 
that is correct to 18457734525360901453873570 decimal digits
is given by
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(1)
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found by R. Sabey in 2004 (Friedman 2004).
Castellanos (1988ab) gives several curious approximations to 
,
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(2)
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(3)
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(4)
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(5)
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(6)
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(7)
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which are good to 6, 7, 9, 10, 12, and 15 digits respectively.
E. Pegg Jr. (pers. comm., Mar. 2, 2002), found
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(8)
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which is good to 7 digits.
J. Lafont (pers. comm., MAy 16, 2008) found
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(9)
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where 
is a harmonic number, which is good to 7 digits.
N. Davidson (pers. comm., Sept. 7, 2004) found
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(10)
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which is good to 6 digits.
D. Barron noticed the curious approximation
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(11)
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where 
is Catalan's constant and 
is the Euler-Mascheroni
constant, which however, is only good to 3 digits.
