A361511 a(1) = 1. Thereafter if a(n-1) is a novel term, a(n) = d(a(n-1)); otherwise, if a(n-1) is the t-th non-novel term, a(n) = a(n-1) + d(a(t)), where d is the divisor function A000005.
1, 1, 2, 2, 3, 2, 4, 3, 5, 2, 4, 6, 4, 7, 2, 4, 6, 8, 4, 7, 11, 2, 5, 7, 9, 3, 6, 10, 4, 8, 11, 13, 2, 4, 6, 8, 10, 13, 15, 4, 8, 12, 6, 9, 13, 15, 17, 2, 4, 7, 11, 15, 19, 2, 4, 8, 11, 15, 21, 4, 8, 11, 13, 17, 19, 21, 24, 8, 10, 12, 16, 5, 7, 9, 12, 16, 18, 6, 10, 14, 4, 7, 11, 13, 15, 17, 19, 23
Offset: 1
Examples
The initial terms (in the third column, N = novel term, D = non-novel term): .n.a(n).....t .1,..1,.N, .2,..1,.D,..1 .3,..2,.N, .4,..2,.D,..2 .5,..3,.N, .6,..2,.D,..3 .7,..4,.N, .8,..3,.D,..4 .9,..5,.N, 10,..2,.D,..5 11,..4,.D,..6 12,..6,.N, 13,..4,.D,..7 14,..7,.N, 15,..2,.D,..8 16,..4,.D,..9 17,..6,.D,.10 18,..8,.N, 19,..4,.D,.11 20,..7,.D,.12 21,.11,.N, 22,..2,.D,.13 ... If n=8, for example, a(8) = 3 is a non-novel term, the 4th such, so a(9) = a(8) + d(a(4)) = 3 + d(2) = 5. Comment from _Michael De Vlieger_, Apr 08 2023 (Start) Can be read as an irregular triangle of increasing subsequences: 1; 1, 2; 2, 3; 2, 4; 3, 5; 2, 4, 6; 4, 7; 2, 4, 6, 8; 4, 7, 11; 2, 5, 7, 9; 3, 6, 10; 4, 8, 11, 13; 2, 4, 6, 8, 10, 13, 15; 4, 8, 12; 6, 9, 13, 15, 17; 2, 4, 7, 11, 15, 19; etc. (End) The rows end with the novel terms - see A361512, A361513 - and their lengths are given by A361514.
Links
- Michael De Vlieger, Table of n, a(n) for n = 1..40000
- Michael De Vlieger, Scatterplot of a(n), n = 1..2^16, showing records in red, smallest missing numbers in blue (small until they enter sequence, then large), terms deriving from novel predecessors in gold, otherwise green.
- Michael De Vlieger, Plot that shows the increasing subsequences that form the rows when the sequence is regarded as an irregular triangle
Programs
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Mathematica
nn = 120; c[] = False; f[n] := DivisorSigma[0, n]; a[1] = m = 1; Do[(If[c[#], a[n] = # + f[a[m]] ; m++, a[n] = f[#] ]; c[#] = True) &[a[n - 1]], {n, 2, nn}]; Array[a, nn] (* Michael De Vlieger, Apr 08 2023 *)
Comments