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%I #19 Nov 09 2020 02:56:38
%S 0,1,2,3,3,5,5,6,5,7,5,9,6,7,5,9,9,11,7,7,9,12,9,11,8,8,6,7,10,9,12,9,
%T 16,10,10,7,12,12,14,9,13,10,13,8,9,14,14,15,7,11,11,15,10,11,12,15,
%U 13,11,12,15,16,12,13,13,17,11,13,14,17,12,14,15,18,11,14,15,20,13,14,15,21,15
%N a(n) = (number of times a(n-1) has already appeared) + (number of times a(n-2) has already appeared) + (number of times a(n-3) has already appeared), starting with a(n) = n for n<3.
%C Generalizes A316774, which looks at the frequencies of the two previous terms. Here we look at three previous terms.
%C If we look at just one previous term, we get 0, 1, 1, 2, 1, 3, 1, 4, 1, 5, 1, 6, 1, 7, 1, 8, ..., which is A133622 prefixed by 0, 1, or A152271 with its initial 1 changed to 0.
%H N. J. A. Sloane, <a href="/A330332/b330332.txt">Table of n, a(n) for n = 0..10000</a>
%H Rémy Sigrist, <a href="/A330332/a330332.png">Density plot of the first 2^22 terms</a>
%p b:= proc() 0 end:
%p a:= proc(n) option remember; local t;
%p t:= `if`(n<3, n, b(a(n-1))+b(a(n-2))+b(a(n-3)));
%p b(t):= b(t)+1; t
%p end:
%p [seq(a(n), n=0..200)]; # Following _Alois P. Heinz_'s program for A316774
%t b[_] = 0;
%t a[n_] := a[n] = Module[{t}, t = If[n<3, n, b[a[n-1]] + b[a[n-2]] + b[a[n-3]]]; b[t]++; t];
%t a /@ Range[0, 200] (* _Jean-François Alcover_, Nov 09 2020, after Maple *)
%Y Cf. A316774, A133622, A152271.
%K nonn,look
%O 0,3
%A _N. J. A. Sloane_, Dec 14 2019
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