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 A019553 a(n) is the concatenation of n and 5n. 9
 15, 210, 315, 420, 525, 630, 735, 840, 945, 1050, 1155, 1260, 1365, 1470, 1575, 1680, 1785, 1890, 1995, 20100, 21105, 22110, 23115, 24120, 25125, 26130, 27135, 28140, 29145, 30150, 31155, 32160, 33165, 34170, 35175, 36180, 37185, 38190, 39195, 40200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are divisible by 15. - Michel Marcus, 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(5*n) + 1) + 5*n, with n >= 1. - Paolo P. Lava, Mar 24 2010 MAPLE a:=n->n*10^floor(log10(5*n)+1)+5*n: seq(a(n), n=1..50); # Muniru A Asiru, Jun 23 2018 MATHEMATICA n5n[n_]:=Module[{n5=5n}, n*10^IntegerLength[n5]+n5]; Array[n5n, 40] (* Harvey P. Dale, Apr 08 2012 *) nxt[n_]:=Module[{idn=IntegerDigits[n], idn5=IntegerDigits[5n]}, FromDigits[Join[idn, idn5]]]; Array[nxt, 100] (* Vincenzo Librandi, Feb 04 2014 *) PROG (MAGMA) [Seqint(Intseq(5*n) cat Intseq(n)): n in [1..50]]; // Vincenzo Librandi, Feb 04 2014 (PARI) a(n) = eval(Str(n, 5*n)); \\ Michel Marcus, Sep 21 2015 CROSSREFS Cf. concatenation of n and k*n: A020338 (k=1), A019550 (k=2), A019551 (k=3), A019552 (k=4), this sequence (k=5), A009440 (k=6), A009441 (k=7), A009470 (k=8), A009474 (k=9). Sequence in context: A028230 A122572 A067560 * A234249 A112496 A000483 Adjacent sequences:  A019550 A019551 A019552 * A019554 A019555 A019556 KEYWORD nonn,base,easy,less AUTHOR R. Muller STATUS approved

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Last modified October 17 15:01 EDT 2019. Contains 328116 sequences. (Running on oeis4.)