cp's OEIS Frontend

A121993 Numbers k that yield a smaller number a(k) under the "Look and Say" function A045918.

%I A121993 #16 Aug 28 2025 17:24:02
%S A121993 33,44,55,66,77,88,99,111,222,333,444,555,666,777,888,999,1111,1222,
%T A121993 1333,1444,1555,1666,1777,1888,1999,2000,2111,2222,2233,2244,2255,
%U A121993 2266,2277,2288,2299,2333,2444,2555,2666,2777,2888,2999,3000,3111,3222,3300,3311
%N A121993 Numbers k that yield a smaller number a(k) under the "Look and Say" function A045918.
%H A121993 Reinhard Zumkeller, <a href="/A121993/b121993.txt">Table of n, a(n) for n = 1..10000</a>
%H A121993 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/LookandSaySequence.html">Look and Say Sequence</a>.
%e A121993 a(26)=2000 because under the Look and Say operator, 2000 is described as one two three zeros or: 1230, which is smaller than 2000.
%o A121993 (Haskell)
%o A121993 a121993 n = a121993_list !! (n-1)
%o A121993 a121993_list = filter (\x -> a045918 x < x) [0..]
%o A121993 -- _Reinhard Zumkeller_, Jan 25 2014
%o A121993 (Python)
%o A121993 from itertools import groupby
%o A121993 def ok(n): return n > int(''.join(str(len(list(g)))+k for k, g in groupby(str(n))))
%o A121993 print([k for k in range(3312) if ok(k)]) # _Michael S. Branicky_, May 26 2023
%Y A121993 Cf. A045918, A005150, A005341, A023989, A079475.
%K A121993 easy,nonn,base,changed
%O A121993 1,1
%A A121993 _Sergio Pimentel_, Sep 11 2006