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A020338
Doublets: base-10 representation is the juxtaposition of two identical strings.
60
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
OFFSET
1,1
LINKS
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).
Sequence in context: A056524 A110745 A280826 * A052191 A321536 A110732
KEYWORD
nonn,base,look
STATUS
approved