A022515 - OEIS

%I #15 Oct 06 2019 16:55:15

%S 4,41,1141,114112,21124112,211241211221,112212211141211221,

%T 1122122111411322112212,21112212223112411322112212,

%U 21112212223112412112312311221321

%N Describe previous term from the right (method B - initial term is 4).

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

%H Reinhard Zumkeller, <a href="/A022515/b022515.txt">Table of n, a(n) for n = 0..21</a>

%e The term after 1141 is obtained by saying "1 once, 4 once, 1 twice", which gives 114112.

%t A022515[1]:=4;A022515[n_]:=A022515[n]=FromDigits[Flatten[{First[#],Length[#]}&/@Split[Reverse[IntegerDigits[A022515[n-1]]]]]];Map[A022515,Range[15]] (* _Peter J. C. Moses_, Apr 22 2013 *)

%o (Haskell)

%o import Data.List (group, transpose)

%o a022515 n = a022515_list !! n

%o a022515_list = 4 : f [4] :: [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. A022481, A022488, A022514-A022520.

%K nonn,base,easy,nice

%O 0,1

%A _N. J. A. Sloane_

%E More terms from _Patrick De Geest_, Jun 15 1999