A204846 - OEIS

A204846

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

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

1,4

LINKS

Table of n, a(n) for n=1..67.

Samuel Yates, The Mystique of Repunits, Math. Mag. 51 (1978), 22-28.

EXAMPLE

Triangle begins:

11;

3,11,37;

11,101;

3,37;

11,41,271;

...

MATHEMATICA

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 *)

CROSSREFS

Cf. A002275, A102380, A204845.

Sequence in context: A214676 A010198 A307245 * A127991 A110305 A010199

Adjacent sequences: A204843 A204844 A204845 * A204847 A204848 A204849

KEYWORD

nonn,tabf

AUTHOR

N. J. A. Sloane, Jan 19 2012

EXTENSIONS

More terms from Amiram Eldar, May 11 2024

STATUS

approved