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 A270667 #13 Apr 02 2020 03:01:58
%S A270667 1,21,39,57,91,93,111,129,133,183,201,217,219,237,247,259,273,291,301,
%T A270667 309,327,381,399,403,417,427,453,469,471,481,489,511,543,553,559,579,
%U A270667 589,597,633,651,669,679,687,703,721,723,741,763,777,793,813,817,831,849,871,889,903,921,939,949,973,993
%N A270667 Nonprime and squarefree Löschian numbers (A003136).
%C A270667 5187 is the first term that has 4 prime divisors.
%H A270667 Amiram Eldar, <a href="/A270667/b270667.txt">Table of n, a(n) for n = 1..10000</a>
%e A270667 21 is a term because 21 = 3*7 = 4^2 + 4*1 + 1^2.
%t A270667 Select[Range[10^3], And[SquareFreeQ@ #, ! PrimeQ@ #, Resolve[Exists[{x, y}, Reduce[# == x^2 + x y + y^2, {x, y}, Integers]]]] &] (* _Michael De Vlieger_, Mar 21 2016, after _Jean-François Alcover_ at A003136 *)
%o A270667 (PARI) x='x+O('x^1000); p=eta(x)^3/eta(x^3); for(n=0, 999, if(polcoeff(p, n) != 0 && issquarefree(n) && !isprime(n), print1(n, ", ")));
%Y A270667 Cf. A003136, A005117, A034021.
%K A270667 nonn
%O A270667 1,2
%A A270667 _Altug Alkan_, Mar 21 2016