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 A293521 #17 Mar 14 2025 06:17:51
%S A293521 0,1,1,3,3,2,5,9,8,11,15,24,30,42,51,76,94,126,158,217,298,403,539,
%T A293521 731,970,1305,1748,2322,3179,4225,5715,7596,10259,13731,18357,24771,
%U A293521 33184,44448,59968,80764,107973,145638,195237,262446,352904,474964,637081,856232,1149966,1543986,2076534,2789516
%N A293521 Number of surviving (but not bifurcating) nodes at generation n in the binary tree of persistently squarefree numbers (see A293230).
%F A293521 a(n) = Sum_{k=(2^n)..(2^(1+n)-1)} abs(A293233(k)) * [1 == A008966(2*k)+A008966(1+2*k)].
%F A293521 a(n) = A293518(n) + A293519(n). [even survivors + odd survivors.]
%e A293521 a(2) = 1 because in the binary tree illustrated in A293230, there is only one node at the level (namely, the node 6) that spawns just one offspring.
%o A293521 (PARI) \\ See program at A293520.
%Y A293521 Cf. A293230, A293441, A293518, A293519, A293520, A293522.
%K A293521 nonn
%O A293521 0,4
%A A293521 _Antti Karttunen_, Oct 12 2017