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 A081390 #9 Jul 22 2024 15:23:59 %S A081390 6,10,12,15,16,20,21,22,27,28,29,30,32,33,34,36,37,39,53,54,55,56,57, %T A081390 58,65,67,79,80,109,110,129,135,159,161,170,171,255,783,784,785,786, %U A081390 902 %N A081390 Number k such that the k-th Catalan number has only one non-unitary prime divisor; all the other prime divisors have exponent one. %C A081390 a(43) > 25000, if it exists. - _Amiram Eldar_, Jul 22 2024 %e A081390 902 is a term because binomial(1804,902)/903 has 189 prime factor, 188 stand with exponent one, but 2 with 5: 2^5. %t A081390 c[n_] := Count[FactorInteger[n][[;; , 2]], _?(# > 1 &)]; Select[Range[0, 1000], c[CatalanNumber[#]] == 1 &] (* _Amiram Eldar_, Jul 22 2024 *) %Y A081390 Cf. A000108, A081386, A081387, A081388, A081389, A080405, A080664. %K A081390 nonn,more %O A081390 1,1 %A A081390 _Labos Elemer_, Mar 27 2003