A121993 Numbers k that yield a smaller number a(k) under the "Look and Say" function A045918.
33, 44, 55, 66, 77, 88, 99, 111, 222, 333, 444, 555, 666, 777, 888, 999, 1111, 1222, 1333, 1444, 1555, 1666, 1777, 1888, 1999, 2000, 2111, 2222, 2233, 2244, 2255, 2266, 2277, 2288, 2299, 2333, 2444, 2555, 2666, 2777, 2888, 2999, 3000, 3111, 3222, 3300, 3311
Offset: 1
Examples
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.
Links
- Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
- Eric Weisstein's World of Mathematics, Look and Say Sequence.
Programs
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Haskell
a121993 n = a121993_list !! (n-1) a121993_list = filter (\x -> a045918 x < x) [0..] -- Reinhard Zumkeller, Jan 25 2014
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Python
from itertools import groupby def ok(n): return n > int(''.join(str(len(list(g)))+k for k, g in groupby(str(n)))) print([k for k in range(3312) if ok(k)]) # Michael S. Branicky, May 26 2023
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