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 A020338 Doublets: base-10 representation is the juxtaposition of two identical strings. 51
 11, 22, 33, 44, 55, 66, 77, 88, 99, 1010, 1111, 1212, 1313, 1414, 1515, 1616, 1717, 1818, 1919, 2020, 2121, 2222, 2323, 2424, 2525, 2626, 2727, 2828, 2929, 3030, 3131, 3232, 3333, 3434, 3535, 3636, 3737, 3838, 3939, 4040, 4141, 4242, 4343, 4444, 4545, 4646 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS R. Zumkeller, Table of n, a(n) for n = 1..9999 Eric Weisstein's World of Mathematics, Concatenation FORMULA a(n) = n*10^(A004216(n)+1) + n. - Reinhard Zumkeller, Aug 11 2007 G.f.: 11*x/(1-x)^2 - Sum_{d >= 1} 9*x^(10^d)*(100^d*x-10^d*x-100^d)/(1-x)^2). - Robert Israel, Jan 02 2015 a(n) = n || n. (Where "||" denotes "concatenate". See link "Concatenation".) - Halfdan Skjerning, Apr 01 2018 MAPLE seq(seq((10^d+1)*n, n = 10^(d-1)..10^d-1), d=1..3); # Robert Israel, Jan 02 2015 MATHEMATICA nxt[n_]:=Module[{idn=IntegerDigits[n], idn1=IntegerDigits[n]}, FromDigits[Join[idn, idn1]]]; Array[nxt, 100] (* Vincenzo Librandi, Feb 04 2014 *) PROG (MAGMA) [Seqint(Intseq(n) cat Intseq(n)): n in [1..46]]; // Bruno Berselli, Mar 20 2013 (Haskell) a020338 n = read (ns ++ ns) :: Integer  where ns = show n -- Reinhard Zumkeller, Jun 07 2015 (PARI) a(n) = eval(Str(n, n)); \\ Michel Marcus, Sep 10 2015 (Sage) [int(str(n)+str(n)) for n in range(1, 47)] # Danny Rorabaugh, Oct 10 2015 (GAP) Flat(List([1..2], d->List([10^(d-1)..10^d-1], n->(10^d+1)*n))); # Muniru A Asiru, Mar 31 2018 CROSSREFS Cf. concatenation of n and k*n: this sequence (k=1), A019550 (k=2), A019551 (k=3), A019552 (k=4), A019553 (k=5), A009440 (k=6), A009441 (k=7), A009470 (k=8), A009474 (k=9). Cf. A004216, A056524. Sequence in context: A056524 A110745 A280826 * A052191 A321536 A110732 Adjacent sequences:  A020335 A020336 A020337 * A020339 A020340 A020341 KEYWORD nonn,base,look AUTHOR STATUS approved

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Last modified September 20 16:46 EDT 2019. Contains 327242 sequences. (Running on oeis4.)