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A127421 Numbers whose decimal expansion is a concatenation of 2 consecutive increasing nonnegative numbers. 9
1, 12, 23, 34, 45, 56, 67, 78, 89, 910, 1011, 1112, 1213, 1314, 1415, 1516, 1617, 1718, 1819, 1920, 2021, 2122, 2223, 2324, 2425, 2526, 2627, 2728, 2829, 2930, 3031, 3132, 3233, 3334, 3435, 3536, 3637, 3738, 3839, 3940, 4041, 4142, 4243, 4344, 4445, 4546 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Primes in the sequence are in A030458. - Bruno Berselli, Mar 25 2015

a(n) always has an even number of digits unless n is in A103456. - Alonso del Arte, Oct 30 2019

LINKS

N. J. A. Sloane, Table of n, a(n) for n = 1..25000

FORMULA

a(n) = n*10^(1 + floor(log(n+1))) + n + 1, with n >= 0. - Paolo P. Lava, May 09 2008

EXAMPLE

a(1) = "0,1" = 1.

a(13) = "12,13" = 1213.

MAPLE

a:= n-> parse(cat(n-1, n)):

seq(a(n), n=1..55);  # Alois P. Heinz, Jul 05 2018

MATHEMATICA

nMax = 49; digitsList = IntegerDigits[Range[0, nMax]]; Table[FromDigits[Flatten[{digitsList[[n]], digitsList[[n + 1]]}]], {n, nMax - 1}] (* Alonso del Arte, Oct 24 2019 *)

PROG

(MAGMA) [Seqint(Intseq(n+1) cat Intseq(n)): n in [0..50]]; // Bruno Berselli, Mar 25 2015

(Python) for n in range(100): print(int(str(n)+str(n+1))) # David F. Marrs, Sep 17 2018

(Scala) val numerStrs = (0 to 49).map(Integer.toString(_)).toList

val concats = (numerStrs.dropRight(1)) zip (numerStrs.drop(1))

concats.map(x => Integer.parseInt(x._1 + x._2)) // Alonso del Arte, Oct 24 2019

CROSSREFS

A variant of A001704.

For concatenations of exactly k consecutive integers see A000027 (k = 1), A127421 (k = 2), A001703 (k = 3), A279204 (k = 4). For 2 or more see A035333.

Cf. A030458, A127423, A127424, A127425, A103456.

Sequence in context: A072485 A035333 A001704 * A112131 A233032 A088997

Adjacent sequences:  A127418 A127419 A127420 * A127422 A127423 A127424

KEYWORD

nonn,base,easy

AUTHOR

Artur Jasinski, Jan 14 2007

EXTENSIONS

More terms from Joshua Zucker, May 15 2007

STATUS

approved

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Last modified December 13 20:27 EST 2019. Contains 329973 sequences. (Running on oeis4.)