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A006751 Describe the previous term! (method A - initial term is 2).
(Formerly M2052)
27
2, 12, 1112, 3112, 132112, 1113122112, 311311222112, 13211321322112, 1113122113121113222112, 31131122211311123113322112, 132113213221133112132123222112, 11131221131211132221232112111312111213322112, 31131122211311123113321112131221123113111231121123222112 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Method A = 'frequency' followed by 'digit'-indication.

Contribution from Carmine Suriano, Sep 07 2010:  No digit exceeds 3. If the starting number a(1) is a single-digit number greater than 3 this will remain as the last digit, all the remaining in any term being no greater than 3.

a(n) = value of concatenation of n-th row in A088203. - Reinhard Zumkeller, Aug 09 2012

This is because for all n > 1, a(n) begins by 1 or 3 and ends by 2. [Jean-Christophe Hervé, May 07 2013]

REFERENCES

J. H. Conway, The weird and wonderful chemistry of audioactive decay, in T. M. Cover and Gopinath, eds., Open Problems in Communication and Computation, Springer, NY 1987, pp. 173-188.

S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 452-455.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

I. Vardi, Computational Recreations in Mathematica. Addison-Wesley, Redwood City, CA, 1991, p. 4.

LINKS

T. D. Noe, Table of n, a(n) for n=1..20

S. R. Finch, Conway's Constant

Eric Weisstein's World of Mathematics, Look and Say Sequence

FORMULA

a(n+1) = A045918(a(n)). - Reinhard Zumkeller, Aug 09 2012

EXAMPLE

E.g. the term after 3112 is obtained by saying "one 3, two 1's, one 2", which gives 132112.

MATHEMATICA

RunLengthEncode[ x_List ] := (Through[ {First, Length}[ #1 ] ] &) /@ Split[ x ]; LookAndSay[ n_, d_:1 ] := NestList[ Flatten[ Reverse /@ RunLengthEncode[ # ] ] &, {d}, n - 1 ]; F[ n_ ] := LookAndSay[ n, 2 ][ [ n ] ]; Table[ FromDigits[ F[ n ] ], {n, 1, 11} ] (* Zerinvary Lajos, Mar 21 2007 *)

PROG

(Haskell)

a006751 = foldl1 (\v d -> 10 * v + d) . map toInteger . a088203_row

-- Reinhard Zumkeller, Aug 09 2012

(Perl)

# This outputs the first n elements of the sequence, where n is given on the command line.

$s = 2;

for (2..shift @ARGV) {

    print "$s, ";

    $s =~ s/(.)\1*/(length $&).$1/eg;

}

print "$s\n";

## Arne 'Timwi' Heizmann (timwi(AT)gmx.net), Mar 12 2008)

CROSSREFS

Cf. A001155, A005150, A006715, A001140, A001141, A001143, A001145, A001151, A001154.

Cf. A088203 (continuous version).

Sequence in context: A058975 A057120 A112512 * A023989 A001389 A022914

Adjacent sequences:  A006748 A006749 A006750 * A006752 A006753 A006754

KEYWORD

nonn,base,easy,nice

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified December 22 04:20 EST 2014. Contains 252328 sequences.