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A120576
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Irregular array where the n-th row are the divisors, not occurring earlier in the sequence, of the sum of the terms in all previous rows. a(1)=2.
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4
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2, 1, 3, 6, 4, 12, 7, 14, 28, 11, 77, 5, 15, 33, 55, 165, 73, 146, 219, 438, 9, 18, 657, 1314, 8, 16, 23, 24, 36, 46, 48, 69, 72, 92, 138, 144, 184, 207, 276, 368, 414, 552, 828, 1104, 1656, 3312, 1847, 12929, 5541, 9235, 27705, 19, 38, 3694, 35093, 70186, 487, 974
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OFFSET
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1,1
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COMMENTS
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Is this sequence a permutation of the positive integers?
Length of rows varies widely, is often 1. Row 129 has 12236 terms. - Michael De Vlieger, Oct 03 2017
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LINKS
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EXAMPLE
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Array begins:
2
1
3
6
4,12
7,14,28
Now these terms add up to 77. So row 7 is the divisors of 77 which do not occur earlier in the sequence. 1 and 7 occur in earlier rows, so row 7 is (11,77).
Lengths of rows of a(n) and relation to number of divisors of the sum of terms in all previous rows.
Key: n = index; m = length of row n; k = sum of the terms in all previous rows.
tau(k(n-1)) = number of divisors of k of the previous row.
delta = tau(k(n-1)) - m: i.e., divisors of k(n - 1) not in row n of A120576.
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n tau(k(n-1)) m delta k
--------------------------------------
1 2 2 0 2
2 2 1 1 3
3 2 1 1 6
4 4 1 3 12
5 6 2 4 28
6 6 3 3 77
7 4 2 2 165
8 8 5 3 438
9 8 4 4 1314
10 12 4 8 3312
11 30 22 8 12929
12 4 2 2 27705
13 8 3 5 70186
14 8 5 3 179216
15 20 10 10 541544
16 16 8 8 1559024
17 20 11 9 4603588
18 24 17 7 13776209
19 2 1 1 27552418
20 4 1 3 55104836
...
(End)
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MATHEMATICA
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f[t_] := Flatten[Append[t, Select[Divisors[Plus @@ t], FreeQ[t, # ] &]]]; Nest[f, {2}, 14] (* Ray Chandler, Jun 17 2006 *)
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CROSSREFS
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KEYWORD
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nonn,tabf
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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