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A204846
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Irregular triangle read by rows in which row n lists algebraic prime factors of the repunit (10^n - 1)/9 (A002275(n)).
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5
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1, 1, 1, 11, 1, 3, 11, 37, 1, 11, 101, 3, 37, 11, 41, 271, 1, 3, 7, 11, 13, 37, 101, 1, 11, 239, 4649, 3, 37, 41, 271, 11, 73, 101, 137, 1, 3, 7, 11, 13, 37, 333667, 1, 11, 41, 101, 271, 9091, 3, 37, 239, 4649, 11, 21649, 513239, 1, 3, 7, 11, 13, 37, 73, 101, 137, 9901, 41, 271
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OFFSET
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1,4
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LINKS
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EXAMPLE
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Triangle begins:
1;
1;
1;
11;
1;
3,11,37;
1;
11,101;
3,37;
11,41,271;
...
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MATHEMATICA
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rows[nmax_] := Module[{prim = {1}, r = {{1}}, p, c}, Do[p = FactorInteger[(10^n - 1)/9][[;; , 1]]; c = Complement[p, Complement[p, prim]]; If[c == {}, c = {1}]; AppendTo[r, c]; prim = Union[prim, p], {n, 2, nmax}]; r]; rows[25] // Flatten (* Amiram Eldar, May 11 2024 *)
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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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