In mathematics, Somos' quadratic recurrence constant, named after Michael Somos, is the number

${\displaystyle \sigma ={\sqrt {1{\sqrt {2{\sqrt {3\cdots }}}}}}=1^{1/2}\;2^{1/4}\;3^{1/8}\cdots .\,}$

This can be easily re-written into the far more quickly converging product representation

${\displaystyle \sigma =\sigma ^{2}/\sigma =\left({\frac {2}{1}}\right)^{1/2}\left({\frac {3}{2}}\right)^{1/4}\left({\frac {4}{3}}\right)^{1/8}\left({\frac {5}{4}}\right)^{1/16}\cdots .}$

The constant σ arises when studying the asymptotic behaviour of the sequence

${\displaystyle g_{0}=1\,;\,g_{n}=ng_{n-1}^{2},\qquad n>1,\,}$

with first few terms 1, 1, 2, 12, 576, 1658880 ... (sequence A052129 in the OEIS). This sequence can be shown to have asymptotic behaviour as follows:[1]

${\displaystyle g_{n}\sim {\frac {\sigma ^{2^{n}}}{n+2+O({\frac {1}{n}})}}.}$

Guillera and Sondow give a representation in terms of the derivative of the Lerch transcendent:

${\displaystyle \ln \sigma ={\frac {-1}{2}}{\frac {\partial \Phi }{\partial s}}\left({\frac {1}{2}},0,1\right)}$

where ln is the natural logarithm and ${\displaystyle \Phi }$(zsq) is the Lerch transcendent.

Using series acceleration it is the sum of the n-th differences of ln(k) at k=1 as given by:

${\displaystyle \ln \sigma =\sum _{n=1}^{\infty }\sum _{k=0}^{n}(-1)^{n-k}{n \choose k}\ln(k+1).}$

Finally,

${\displaystyle \sigma =1.661687949633594121296\dots \;}$ (sequence A112302 in the OEIS).