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%I #21 Jul 07 2017 17:24:51
%S 1,11,12,2111,1321,11213111,1331112112,211221133211,12213212221221,
%T 11221123112131112211,122213311121123121122212,
%U 211123122111312112211332112311
%N Describe previous term from the right (method B - initial term is 1).
%C Method B = 'digit'-indication followed by 'frequency'.
%H Reinhard Zumkeller, <a href="/A022481/b022481.txt">Table of n, a(n) for n = 1..24</a>
%e E.g. the term after 1321 is obtained by saying "1 once, 2 once, 3 once, 1 once", which gives 11213111.
%t A022481[1]:=1;A022481[n_]:=A022481[n]=FromDigits[Flatten[{First[#],Length[#]}&/@Split[Reverse[IntegerDigits[A022481[n-1]]]]]];Map[A022481,Range[15]] (* _Peter J. C. Moses_, Apr 22 2013 *)
%t a[1]=1;a[n_]:=a[n]=FromDigits[Flatten[Replace[ Replace[ Replace[ Split[ Reverse[IntegerDigits[a[n-1]]]],{x_,y_}->{x,Length[{x,y}]},{1}],{x_,y_,z_}->{x,Length[{x,y,z}]},{1}],{x_}->{x,Length[{x}]},{1}]]];Array[a,15] (* _Ivan N. Ianakiev_, Jun 22 2017 *)
%t NestList[FromDigits@ Flatten@ Reverse@ Map[{First@ #, Length@ #} &, Split@ IntegerDigits@ #] &, 1, 11] (* _Michael De Vlieger_, Jun 26 2017 *)
%o (Haskell)
%o import Data.List (group, transpose)
%o a022481 n = a022481_list !! (n-1)
%o a022481_list = 1 : f [1] :: [Integer] where
%o f xs = (read $ concatMap show ys) : f ys where
%o ys = concat $ transpose [map head zss, map length zss]
%o zss = reverse $ group xs
%o -- _Reinhard Zumkeller_, Jan 26 2014
%Y Cf. A022488, A022514, A022515, A022516, A022517, A022518, A022519, A022520.
%K nonn,base,easy,nice
%O 1,2
%A _Clark Kimberling_
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