Catalan's constant
In mathematics, Catalan's constant , is defined by where is the Dirichlet beta function. Its numerical value is approximately It is not known whether is irrational, let alone transcendental. has been called "arguably the most basic constant whose irrationality and transcendence (though strongly suspected) remain unproven". Catalan's constant was named after Eugène Charles Catalan, who found quickly-converging series for its calculation, and published a memoir on it in 1865. Uses. In low-dimensional topology, Catalan's constant is 1/4 of the volume of an ideal hyperbolic octahedron, and therefore 1/4 of the hyperbolic volume of  the complement of the Whitehead link. It is 1/8 of the volume of the complement of the Borromean rings. In combinatorics and statistical mechanics, it arises in connection with counting domino tilings, spanning trees, and Hamiltonian cycles of grid graphs. In number theory, Catalan's constant appears in a conjectured formula for the asymptotic number of primes of the form formula_2 according to Hardy and Littlewood's Conjecture F. However, it is an unsolved problem (one of Landau's problems) whether there are even infinitely many primes of this form. Catalan's constant also appears in the calculation of the  mass distribution of spiral galaxies. Known digits. The number of known digits of Catalan's constant has increased dramatically during the last decades. This is due both to the increase of performance of computers as well as to algorithmic improvements. Integral identities. As Seán Stewart writes, "There is a rich and seemingly endless source of definite integrals that can be equated to or expressed in terms of Catalan's constant." Some of these expressions include: formula_3 where the last three formulas are related to Malmsten's integrals. If is the complete elliptic integral of the first kind, as a function  of the elliptic modulus , then formula_4 If is the complete elliptic integral of the second kind, as a function of the elliptic modulus , then formula_5 With the gamma function formula_6 The integral formula_7 is a known special function, called the inverse tangent integral, and was extensively studied by Srinivasa Ramanujan. Relation to other special functions. appears in values of the second polygamma function, also called the trigamma function, at fractional arguments: formula_8 Simon Plouffe gives an infinite collection of identities between the trigamma function, and Catalan's constant; these are expressible as paths on a graph. Catalan's constant occurs  frequently in relation to the Clausen function, the inverse tangent integral, the inverse sine integral, the Barnes -function, as well as integrals and series summable in terms of the aforementioned functions. As a particular example, by first expressing the inverse tangent integral in its closed form – in terms of Clausen functions – and then expressing those Clausen functions in terms of the Barnes -function, the following expression is obtained (see Clausen function for more): formula_9 If one defines the Lerch transcendent (related to the Lerch zeta function) by formula_10 then formula_11 Quickly converging series. The following two formulas involve quickly converging  series, and are thus appropriate for numerical computation: formula_12 and formula_13 The theoretical foundations for such series are given by Broadhurst, for the first formula, and Ramanujan, for the second formula. The algorithms for fast evaluation of the Catalan constant were constructed by E. Karatsuba. Using these series, calculating Catalan's constant is now about as fast as calculating Apery's constant, formula_14. Other quickly converging series, due to Guillera and Pilehrood and employed by the y-cruncher software, include: formula_15 formula_16 formula_17 All of these series have time complexity formula_18. Continued fraction. can be expressed in the following form