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 A130856 #14 Nov 28 2022 09:59:39 %S A130856 1,1,2,5,5,6,6,6,8,2,2,7,4,4,9,9,9,6,6,6,3,3,3,9,9,9,8,2,2,4,1,1,3,3, %T A130856 3,1,7,7,2,5,5,9,9,9,9,9,9,9,9,9,9,9,9,6,6,6,3,3,3,9,9,9,3,3,3,6,6,6, %U A130856 9,9,9,9,9,9,9,9,9,9,9,9,8,2,2,4,1,1,3 %N A130856 The digital root (A010888) of the Catalan numbers A000108. %H A130856 Nathaniel Johnston, <a href="/A130856/b130856.txt">Table of n, a(n) for n = 0..10000</a> %p A130856 A130856 := proc(n) option remember: if(n=0)then return 1:fi: return ((add(procname(k)*procname(n-1-k),k=0..n-1)-1) mod 9) + 1 end: seq(A130856(n),n=0..100); # _Nathaniel Johnston_, May 05 2011 %t A130856 droot[n_]:=NestWhile[Total[IntegerDigits[#]]&,n,#>9&]; droot/@CatalanNumber[ Range[ 0,100]] (* _Harvey P. Dale_, Apr 09 2022 *) %o A130856 (PARI) a(n) = (binomial(2*n,n)/(n+1)-1)%9 + 1; \\ _Michel Marcus_, Nov 28 2022 %Y A130856 Cf. A000108, A010888, A130851. %K A130856 nonn,easy,base %O A130856 0,3 %A A130856 _N. J. A. Sloane_, Aug 21 2007 %E A130856 Name corrected by _Nathaniel Johnston_, May 05 2011