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A204846 Irregular triangle read by rows in which row n lists algebraic prime factors of the repunit (10^n - 1)/9 (A002275(n)).

%I A204846 #17 May 11 2024 09:30:50
%S A204846 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,
%T A204846 239,4649,3,37,41,271,11,73,101,137,1,3,7,11,13,37,333667,1,11,41,101,
%U A204846 271,9091,3,37,239,4649,11,21649,513239,1,3,7,11,13,37,73,101,137,9901,41,271
%N A204846 Irregular triangle read by rows in which row n lists algebraic prime factors of the repunit (10^n - 1)/9 (A002275(n)).
%H A204846 Samuel Yates, <a href="http://www.jstor.org/stable/2689643">The Mystique of Repunits</a>, Math. Mag. 51 (1978), 22-28.
%e A204846 Triangle begins:
%e A204846   1;
%e A204846   1;
%e A204846   1;
%e A204846   11;
%e A204846   1;
%e A204846   3,11,37;
%e A204846   1;
%e A204846   11,101;
%e A204846   3,37;
%e A204846   11,41,271;
%e A204846   ...
%t A204846 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 *)
%Y A204846 Cf. A002275, A102380, A204845.
%K A204846 nonn,tabf
%O A204846 1,4
%A A204846 _N. J. A. Sloane_, Jan 19 2012
%E A204846 More terms from _Amiram Eldar_, May 11 2024