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 A019469 #22 May 28 2021 17:18:03 %S A019469 2,3,4,6,8,9,12,15,16,27,30,32,33,36,39,42,64,81,84,87,90,93,96,108, %T A019469 111,114,117,120,123,128,243,246,249,252,255,256,258,270,273,276,279, %U A019469 282,285,324,327,330,333,336,339 %N A019469 Numbers k such that k does not divide binomial(2*k-4, k-2). %C A019469 Previous name was: Numbers n such that (n-2)-nd Catalan number is not divisible by n. %C A019469 Conjecture (confirmed, see links): sequence is union of powers of two > 1 (A000079) and 3 * A096304. %H A019469 Charles R Greathouse IV, <a href="/A019469/b019469.txt">Table of n, a(n) for n = 1..10000</a> %H A019469 Kevin Ryde, <a href="/A019469/a019469.txt">Proof of Wouter Meeussen's conjectured union</a> %p A019469 A019469:=n->`if`(binomial(2*n-4,n-2) mod n <> 0,n,NULL): seq(A019469(n), n=1..400); # _Wesley Ivan Hurt_, Sep 13 2014 %t A019469 Select[Range[400],!Divisible[Binomial[2#-4,#-2],#]&] (* _Harvey P. Dale_, Aug 13 2015 *) %o A019469 (PARI) valp(n, p)=my(s); while(n\=p, s+=n); s %o A019469 bin(n, p)=valp(2*n, p)-2*valp(n, p) %o A019469 is(n)=my(f=factor(n)); for(i=1, #f~, if(bin(n-2, f[i, 1])<f[i, 2], return(1))); 0 \\ _Charles R Greathouse IV_, Nov 04 2016 %Y A019469 Cf. A000079, A096304, A000984. %Y A019469 Complement of A019470. %K A019469 nonn %O A019469 1,1 %A A019469 _Wouter Meeussen_ %E A019469 Name changed by _Wesley Ivan Hurt_, Sep 16 2014