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A020338
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Doublets: base-10 representation is the juxtaposition of two identical strings.
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60
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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)
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OFFSET
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1,1
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LINKS
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FORMULA
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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
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MAPLE
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seq(seq((10^d+1)*n, n = 10^(d-1)..10^d-1), d=1..3); # Robert Israel, Jan 02 2015
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MATHEMATICA
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nxt[n_]:=Module[{idn=IntegerDigits[n], idn1=IntegerDigits[n]}, FromDigits[Join[idn, idn1]]]; Array[nxt, 100] (* Vincenzo Librandi, Feb 04 2014 *)
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PROG
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(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
(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
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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