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 A019550 a(n) is the concatenation of n and 2n. 11
 12, 24, 36, 48, 510, 612, 714, 816, 918, 1020, 1122, 1224, 1326, 1428, 1530, 1632, 1734, 1836, 1938, 2040, 2142, 2244, 2346, 2448, 2550, 2652, 2754, 2856, 2958, 3060, 3162, 3264, 3366, 3468, 3570, 3672, 3774, 3876, 3978, 4080 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Concatenation of digits of n and 2*n. - Harvey P. Dale, Sep 13 2011 All terms are divisible by 6. - Robert Israel, Sep 21 2015 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Sylvester Smith, A Set of Conjectures on Smarandache Sequences, Bulletin of Pure and Applied Sciences, (Bombay, India), Vol. 15 E (No. 1), 1996, pp. 101-107. FORMULA a(n) = n * 10^floor(log_10(2*n) + 1) + 2*n. - Paolo P. Lava, Mar 24 2010 From Robert Israel, Sep 21 2015 (Start) G.f.: (6*(2*x+75*x^5-60*x^6) + 90*Sum_{k>=1} 10^k*x^(5*10^k)*(5*10^k - (5*10^k-1)*x))/(1-x)^2. a(n+2) - 2*a(n+1) + a(n) = 45*10^(2*k+1) if n = 5*10^k-2, 90*10^k-450*10^(2*k) if n = 5*10^k-1, 0 otherwise. (End) MAPLE seq(n*(10^(1+ilog10(2*n))+2), n=1..100); # Robert Israel, Sep 21 2015 MATHEMATICA nxt[n_]:=Module[{idn=IntegerDigits[n], idn2=IntegerDigits[2n]}, FromDigits[ Join[ idn, idn2]]]; Array[nxt, 40] (* Harvey P. Dale, Sep 13 2011 *) PROG (MAGMA) [Seqint(Intseq(2*n) cat Intseq(n)): n in [1..50]]; // Vincenzo Librandi, Feb 04 2014 (PARI) a(n) = eval(Str(n, 2*n)); \\ Michel Marcus, Sep 21 2015 CROSSREFS Cf. concatenation of n and k*n: A020338 (k=1), this sequence (k=2), A019551 (k=3), A019552 (k=4), A019553 (k=5), A009440 (k=6), A009441 (k=7), A009470 (k=8), A009474 (k=9). Cf. A235497. Sequence in context: A140470 A141766 A069056 * A117304 A022759 A091193 Adjacent sequences:  A019547 A019548 A019549 * A019551 A019552 A019553 KEYWORD nonn,base,less,easy AUTHOR R. Muller EXTENSIONS Offset changed from 0 to 1 by Vincenzo Librandi, Feb 04 2014 STATUS approved

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Last modified May 22 15:13 EDT 2019. Contains 323480 sequences. (Running on oeis4.)