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A007651 Describe the previous term! (method B - initial term is 1).
(Formerly M4768)
21
1, 11, 12, 1121, 122111, 112213, 12221131, 1123123111, 12213111213113, 11221131132111311231, 12221231123121133112213111, 1123112131122131112112321222113113 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

REFERENCES

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

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..25

FORMULA

a(n) = sum{A220424(n,k)*10^(A005341(n)-k): k=1..A005341(n)}. - Reinhard Zumkeller, Dec 15 2012

EXAMPLE

E.g. the term after 1121 is obtained by saying "1 twice, 2 once, 1 once", which gives 122111.

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, 1 ][ [ n ] ]; Table[ FromDigits[ Reverse[ F[ n ] ] ], {n, 1, 15} ]

a[1] = 1; a[n_] := a[n] = FromDigits[Flatten[{First[#], Length[#]}&/@Split[IntegerDigits[a[n-1]]]]]; Map[a, Range[25]] (* Peter J. C. Moses, Mar 22 2013 *)

PROG

(Haskell)

a007651 = foldl1 (\v d -> 10 * v + d) . map toInteger . a220424_row

-- Reinhard Zumkeller, Dec 15 2012

CROSSREFS

Cf. A005150, A022470, A022499, A022500-A022505.

Sequence in context: A087304 A121808 A160265 * A022481 A156338 A272817

Adjacent sequences:  A007648 A007649 A007650 * A007652 A007653 A007654

KEYWORD

nonn,base,easy,nice

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified October 17 16:51 EDT 2019. Contains 328120 sequences. (Running on oeis4.)