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A204845 Irregular triangle read by rows in which row n lists primitive prime factors of the repunit (10^n - 1)/9 (A002275(n)).

%I A204845 #31 May 01 2017 17:47:32
%S A204845 1,11,3,37,101,41,271,7,13,239,4649,73,137,333667,9091,21649,513239,
%T A204845 9901,53,79,265371653,909091,31,2906161,17,5882353,2071723,5363222357,
%U A204845 19,52579,1111111111111111111,3541,27961,43,1933,10838689,23,4093,8779,11111111111111111111111
%N A204845 Irregular triangle read by rows in which row n lists primitive prime factors of the repunit (10^n - 1)/9 (A002275(n)).
%H A204845 Ray Chandler, <a href="/A204845/b204845.txt">Rows n = 1..322, flattened</a> (first 60 rows from Alois P. Heinz)
%H A204845 Samuel Yates, <a href="http://www.jstor.org/stable/2689643">The Mystique of Repunits</a>, Math. Mag. 51 (1978), 22-28.
%e A204845 Triangle begins:
%e A204845 1
%e A204845 11
%e A204845 3 37
%e A204845 101
%e A204845 41 271
%e A204845 7 13
%e A204845 239 4649
%e A204845 73 137
%e A204845 333667
%e A204845 9091
%e A204845 ...
%p A204845 with(numtheory):
%p A204845 S:= proc(n) option remember;
%p A204845       `if`(n=1, {1}, S(n-1) union factorset ((10^n-1)/9))
%p A204845     end:
%p A204845 T:= n-> sort([(S(n) minus `if`(n=1, {}, S(n-1)))[]])[]:
%p A204845 seq(T(n), n=1..30);  # _Alois P. Heinz_, Feb 17 2012
%t A204845 S[n_] := S[n] = If[n==1, {1}, S[n-1] ~Union~ FactorInteger[(10^n-1)/9][[ All, 1]]]; T[n_] := Sort[S[n] ~Complement~ If[n==1, {}, S[n-1]]]; Table[ T[n], {n, 1, 30}] // Flatten (* _Jean-François Alcover_, Mar 13 2017, after _Alois P. Heinz_ *)
%Y A204845 Cf. A002275, A102380, A204846, A204847, A204848.
%K A204845 nonn,tabf
%O A204845 1,2
%A A204845 _N. J. A. Sloane_, Jan 19 2012
%E A204845 More terms from _Alois P. Heinz_, Feb 17 2012