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 A132583 a(n) = n 2's sandwiched between two 1's. 4
 11, 121, 1221, 12221, 122221, 1222221, 12222221, 122222221, 1222222221, 12222222221, 122222222221, 1222222222221, 12222222222221, 122222222222221, 1222222222222221, 12222222222222221, 122222222222222221 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Also, positive numbers each of whose digits indicates its number of adjacent digits (digits on the ends are adjacent to only one while others are adjacent to 2). - Eric Fox, Jul 19 2022 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..500 Index entries for linear recurrences with constant coefficients, signature (11,-10). FORMULA a(0) = 11; a(n) = 10*a(n-1) + 11. - Jonathan Vos Post, Nov 24 2007 [corrected by Jon E. Schoenfield, Jan 28 2018] O.g.f.: 11/((-1+x)*(-1+10*x)) = -(110/9)/(-1+10*x) + (11/9)/(-1+x). - R. J. Mathar, Nov 28 2007 a(n) = 20*(10^n-1)/9 + 10^(n+1) + 1, with n >= 0. - Paolo P. Lava, Dec 19 2007 MAPLE g:=(1+z)/((1-z)*(1-10*z)): gser:=series(g, z=0, 43): seq((coeff(gser, z, n))-1, n=1..24); # Zerinvary Lajos, Feb 25 2009 MATHEMATICA NestList[10#+11&, 11, 20] (* or *) LinearRecurrence[{11, -10}, {11, 121}, 20] (* Harvey P. Dale, Sep 24 2012 *) PROG (Magma) [20*(10^n-1)/9+10^(n+1)+1: n in [0..25]]; // Vincenzo Librandi, Aug 10 2011 CROSSREFS Sequence in context: A106473 A125315 A223676 * A007907 A171285 A216132 Adjacent sequences:  A132580 A132581 A132582 * A132584 A132585 A132586 KEYWORD nonn,base,easy AUTHOR Paul Curtz, Nov 21 2007 STATUS approved

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Last modified August 9 12:05 EDT 2022. Contains 356026 sequences. (Running on oeis4.)