This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A338375 #31 Jan 15 2024 14:57:16 %S A338375 1,1,2,3,3,5,6,7,8,9,11,12,13,15,16,18,19,21,22,24,26,27,29,31,32,34, %T A338375 36,37,39,41,43,45,46,48,50,52,54,56,58,59,61,63,65,67,69,71,73,75,77, %U A338375 79,81,83,85,87,89,91,93,95,97,99,101,104,106,108,110 %N A338375 Number of digits in (2n)! / (2^n * n!). %C A338375 a(n) is the number of digits in double factorial of odd numbers (see A001147). %H A338375 Robert Israel, <a href="/A338375/b338375.txt">Table of n, a(n) for n = 1..10000</a> %F A338375 a(n) = floor(log((2n)!/((2^n)*n!))/log(10))+1. %F A338375 a(n) = A055642(A001147(n)). %e A338375 For n = 7, (2*n-1)!! = 13!! = 135135 and the number of digits is 6. %p A338375 seq(1 + ilog10(doublefactorial(2*n-1)), n=1..100); # _Robert Israel_, Jan 15 2024 %t A338375 a[n_] := IntegerLength[(2 n - 1)!!]; Array[a, 65] (* _Amiram Eldar_, Oct 23 2020 *) %o A338375 (Sage) floor((log(factorial(2 * n) / ((2 ** n) * factorial(n))) / log(10))) + 1 %Y A338375 Cf. A001147, A055642, A068398. %K A338375 nonn,base %O A338375 1,3 %A A338375 _Jeremias M. Gomes_, Oct 23 2020