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A018238 Add 1 to leading digit and put in front. 3
1, 21, 3121, 41213121, 5121312141213121, 61213121412131215121312141213121, 7121312141213121512131214121312161213121412131215121312141213121 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The concatenation of first n terms (if n is small) yields a palindrome: 1, 121, 1213121, etc. - Amarnath Murthy, Apr 08 2003

From M. F. Hasler, May 05 2008: (Start)

This is not the case from n=10 on: According to the formula in A123121 A082215(10) has an even number of digits, the middle digits being "10". (In a strict sense, e.g. Def. 3 of the first reference there, A082215(9) is the last Zimin word on the alphabet {1,...,9}, though.)

While there is less ambiguity about the definition of A018238(10), it is not clear if A018238(11) should start with "11..." or with "10..." (the largest digit of all subsequent terms being "9"). According to the formula in A123121, a(100) has 3 digits more than a(99), so the first choice seems appropriate and has been adopted for the given PARI code.

However, it corresponds to a modified definition, "a(n) = concatenation of n and all preceding terms". a(3) is the only prime term up to a(14) included. The sequence is (1,0,1,0,1,0,...) (mod 3), at least up to a(20). (End)

LINKS

M. F. Hasler, Table of n, a(n) for n=1,...,11.

PROG

(PARI) A018238(n, t="")=for(k=2, n, t=Str(t, k-1, t)); eval(Str(n, t)) \\ M. F. Hasler, May 05 2008

CROSSREFS

Cf. A001511, A082215, A123121.

Sequence in context: A231825 A222092 A222096 * A189447 A098724 A078395

Adjacent sequences:  A018235 A018236 A018237 * A018239 A018240 A018241

KEYWORD

nonn,base

AUTHOR

N. J. A. Sloane, Michael Minic (minic(AT)mtsu.edu)

STATUS

approved

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Last modified October 21 10:33 EDT 2018. Contains 316414 sequences. (Running on oeis4.)