The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A204845 Irregular triangle read by rows in which row n lists primitive prime factors of the repunit (10^n - 1)/9 (A002275(n)). 6
 1, 11, 3, 37, 101, 41, 271, 7, 13, 239, 4649, 73, 137, 333667, 9091, 21649, 513239, 9901, 53, 79, 265371653, 909091, 31, 2906161, 17, 5882353, 2071723, 5363222357, 19, 52579, 1111111111111111111, 3541, 27961, 43, 1933, 10838689, 23, 4093, 8779, 11111111111111111111111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Ray Chandler, Rows n = 1..322, flattened (first 60 rows from Alois P. Heinz) Samuel Yates, The Mystique of Repunits, Math. Mag. 51 (1978), 22-28. EXAMPLE Triangle begins: 1 11 3 37 101 41 271 7 13 239 4649 73 137 333667 9091 ... MAPLE with(numtheory): S:= proc(n) option remember;       `if`(n=1, {1}, S(n-1) union factorset ((10^n-1)/9))     end: T:= n-> sort([(S(n) minus `if`(n=1, {}, S(n-1)))[]])[]: seq(T(n), n=1..30);  # Alois P. Heinz, Feb 17 2012 MATHEMATICA 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 *) CROSSREFS Cf. A002275, A102380, A204846, A204847, A204848. Sequence in context: A298256 A298927 A102380 * A260597 A147555 A157883 Adjacent sequences:  A204842 A204843 A204844 * A204846 A204847 A204848 KEYWORD nonn,tabf AUTHOR N. J. A. Sloane, Jan 19 2012 EXTENSIONS More terms from Alois P. Heinz, Feb 17 2012 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 17 23:03 EDT 2021. Contains 343071 sequences. (Running on oeis4.)