An approximation for the gamma function with is given by
(1)
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where is an arbitrary constant such that ,
(2)
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where is a Pochhammer symbol and
(3)
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and
(4)
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(5)
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with (Lanczos 1964; Luke 1969, p. 30). satisfies
(6)
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and if is a positive integer, then satisfies the identity
(7)
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(Luke 1969, p. 30).
A similar result is given by
(8)
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(9)
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(10)
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where is a Pochhammer symbol, is a factorial, and
(11)
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The first few values of are
(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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(OEIS A054379 and A054380; Whittaker and Watson 1990, p. 253). Note that Whittaker and Watson incorrectly give as 227/60.
Yet another related result gives
(17)
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(Whittaker and Watson 1990, p. 261), where is a Hurwitz zeta function and is a polygamma function.