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 A080405 #15 Apr 19 2014 03:59:21 %S A080405 0,0,1,1,2,3,3,3,4,4,4,5,5,5,6,6,7,8,8,9,9,9,9,9,9,9,9,10,11,11,11,11, %T A080405 12,13,13,13,13,13,13,14,14,15,15,15,16,16,16,17,17,17,17,17,17,19,18, %U A080405 18,18,18,19,19,20,20,20,20,21,22,22,23,22,22,22,23,23,23,24,24,25,25 %N A080405 Number of distinct primes dividing n-th Catalan number. %H A080405 Enrique Pérez Herrero, <a href="/A080405/b080405.txt">Table of n, a(n) for n = 0..3000</a> %F A080405 a(n) = A001221(A000108(n)). %e A080405 a(5) = 3, as C(5) = 42 factors as 2*3*7 (3 distinct prime factors). %t A080405 PrimeNu[CatalanNumber[Range[0,80]]] (* _Harvey P. Dale_, Mar 27 2013 *) %o A080405 (PARI) C(n)=binomial(2*n,n)/(n+1); %o A080405 for(n=1,100,print1(matsize(factor(C(n-1)))[1],", ")); \\ _Joerg Arndt_, Apr 19 2014 %Y A080405 Cf. A001221, A000108. %Y A080405 Cf. A081399. %K A080405 nonn %O A080405 0,5 %A A080405 _Labos Elemer_, Mar 19 2003