# Infinite product with the Catalan numbers

During the analysis of OEIS sequence A179381 I got the following expression with an infinite product:

$$\prod _{k=1}^{\infty } \left(1-4^{-k} C_{k-1}\right)$$

 N[Product[1 - CatalanNumber[k-1]/4^k, {k, 1, Infinity}]]

(* 0.582348 *)


How to calculate this infinite product with higher precision ? With N[..., 10] I got an error message

 (* NIntegrate::ncvb: NIntegrate failed to converge to prescribed accuracy after 9 recursive bisections... *)


I will get the same error message after the transformation to an infinite sum

 N[Exp[Sum[Log[1 - CatalanNumber[k-1]/4^k], {k, 1, Infinity}]], 10]


Does anyone have any idea how to calculate more decimal places?

• This seems to be very slowly convergent. When I manually performed Wynn $\varepsilon$ on your product with 250 digit precision, I was only able to manage the result 0.5829. – J. M. will be back soon Mar 10 '18 at 9:21
• NProduct[1 - CatalanNumber[k - 1]/4^k, {k, 1, Infinity}, PrecisionGoal -> 20] yields 0.582348, but with the warning that "NIntegrate failed to converge". Perhaps increasing MaxRecursion? (as in NProduct[1 - CatalanNumber[k - 1]/4^k, {k, 1, Infinity}, PrecisionGoal -> 20, Method -> {NIntegrate, MaxRecursion -> 25}]) – AccidentalFourierTransform Mar 10 '18 at 18:45

Progress can be made by summing the logs of the terms in the product,

Exp@NSum[Log[1 - CatalanNumber[k - 1]/4^k], {k, 1, Infinity}]


This, in itself, gives the same result as in the question, 0.582348. Moreover, attempting to improve precision by means of the ideas suggested by AccidentalFourierTransform in a comment above are no more successful than they were for NProduct. However, because the terms in the sum approach zero for large k, the terms can be approximated there by

s = Series[Log[1 - CatalanNumber[k - 1]/4^k], {k, Infinity, 8}] // Normal
(* -17/(1024 k^5 π) - 3/(128 k^4 π) - 1/(32 k^3 π) - 25/(512 k^(7/2) Sqrt[π])
- 3/(32 k^(5/2) Sqrt[π]) - 1/(4 k^(3/2) Sqrt[π]) *)


plus many more smaller terms (which are not discarded in the actual computation).

Comparing s with the summand at k == 10^4 shows good agreement.

N[(Log[1 - CatalanNumber[k - 1]/4^k] - s) /. k -> 10^4, 50]
(* -1.5905675495607351158866494466074940463334600411045*10^-37 *)


So, use this approximation to perform the summation for k > 10^4.

suml = NSum[s, {k, 10^4 + 1, Infinity}, WorkingPrecision -> 30,
PrecisionGoal -> 16, Method -> {NIntegrate, MaxRecursion -> 25}]
(* -0.00282091270584364563 *)


The sum for smaller k can, in principle, be performed by NSum using brute force, but it is quite slow. Instead, use

FunctionExpand[CatalanNumber[k - 1]/4^k]
(* Gamma[-(1/2) + k]/(4 Sqrt[π] Gamma[1 + k]) *)


which can be summed much more rapidly.

sums = NSum[Log[1 - Gamma[-(1/2) + k]/(4 Sqrt[π] Gamma[1 + k])], {k, 1, 10^4},
WorkingPrecision -> 30, PrecisionGoal -> 16]
(* -0.5378660936971350 *)


Finally,

Exp[suml + sums]
(* 0.58234803808391506 *)


with an accuracy of about sixteen significant figures. Greater precision can be obtained in a similar manner, although the computations are likely to be much slower.