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%I #10 Apr 14 2023 11:36:03
%S 2,2,3,3,4,4,5,5,6,6,6,7,7,7,7,7,9,9,9,9,9,9,9,9,9,9,9,9,9,9,11,11,11,
%T 11,11,13,13,13,13,13,14,14,14,14,14,14,14,14,14,14,14,14,14,14,14,14,
%U 14,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,17,20,20,20,20,20,20,20,20,21,21
%N a(n) = smallest missing number u in A360179(1..n).
%H Michael De Vlieger, <a href="/A362131/b362131.txt">Table of n, a(n) for n = 1..40000</a>
%H Michael De Vlieger, <a href="/A362131/a362131.png">Labeled scatterplot of A360179(n)</a>, n = 1..300, showing a(n) in small red dots, and A360179(n) in large red when A360179(n) = a(n-1).
%t nn = 120; c[_] := False; q[_] := False; h[_] := 0; f[n_] := DivisorSigma[0, n];
%t a[1] = j = u = w = 1;
%t {2}~Join~Reap[Do[
%t If[c[j],
%t k = j + f[u]; h[j]++; h[u]--,
%t k = f[j]; c[j] = True; h[j]++ ];
%t u = Min[u, j]; Set[{a[n], q[k], j}, {k, True, k}];
%t While[h[u] == 0, u++]; If[k == w, While[q[w], w++]];
%t Sow[w], {n, 2, nn}] ][[-1, -1]]
%Y Cf. A000005, A360179, A362132.
%K nonn
%O 1,1
%A _Michael De Vlieger_, Apr 10 2023
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