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 A073251 #17 Mar 25 2021 04:52:27
%S A073251 33,85,93,141,185,201,213,217,253,265,285,301,321,393,445,453,469,481,
%T A073251 517,533,553,581,589,609,633,669,697,705,713,753,777,789,793,805,813,
%U A073251 869,893,897,901,913,921,933,957,985,993,1001,1005,1041,1045,1065,1113
%N A073251 Numbers k such that k, k+1 and k+2 are nonprime and squarefree.
%C A073251 k-1 and k+3 are not squarefree. Proof: k is odd, otherwise k or k+2 would be divisible by 4. Thus k+1 is even and not divisible by 4, hence k-1 and k+3 are divisible by 4.
%H A073251 Amiram Eldar, <a href="/A073251/b073251.txt">Table of n, a(n) for n = 1..10000</a>
%t A073251 f[upto_]:=Module[{pp=PrimePi[upto],n},lst=Partition[Complement[Range[upto], Prime[Range[pp]]],3,1];Transpose[Select[lst,And@@SquareFreeQ/@#&]][[1]]]; f[1200] (* _Harvey P. Dale_, Mar 21 2011 *)
%o A073251 (PARI) isok1(k) = !isprime(k) && issquarefree(k); \\ A000469
%o A073251 isok(k) = isok1(k) && isok1(k+1) && isok1(k+2); \\ _Michel Marcus_, Mar 25 2021
%Y A073251 Cf. A000469, A005117, A007675, A073249, A073250, A121495.
%K A073251 nonn
%O A073251 1,1
%A A073251 _Reinhard Zumkeller_, Jul 22 2002
%E A073251 Edited by _Klaus Brockhaus_, Aug 07 2006