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 A253953 Numbers that require three steps to collapse to a single digit in base 4 (written in base 4). 3
 223, 1213, 2023, 2122, 2203, 2221, 3133, 11113, 12103, 13033, 20023, 20203, 20221, 21202, 22003, 22021, 22201, 22333, 30313, 31033, 31132, 103033, 110113, 111103, 113032, 121003, 200023, 200203, 200221, 202003, 202021 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS One step consists of taking the number in base 4 and inserting some plus signs between the digits with no restrictions and adding the resulting numbers together in base 4. The numbers given here cannot be taken to a single digit in one or two steps. It is known that three steps always suffice to get to a single digit, and that there are infinitely many numbers that require three steps. LINKS Steve Butler, Table of n, a(n) for n = 1..637 S. Butler, R. Graham and R. Stong, Partition and sum is fast, arXiv:1501.04067 [math.HO], 2014. EXAMPLE As an example a(1)=223 (in base 4).  There are then three ways to insert plus signs in the first step: 2+23   22+3   2+2+3 This gives the numbers (in base 4) as 31, 31, and 13 respectively.  In the second step we have one of the following two: 3+1   1+3 In both cases this gives the number (in base 4) of 10.  Finally in the third step we have the following: 1+0 Which gives 1, a single digit, and we cannot get to a single digit in one or two steps.  (Note, the single digit that we reduce to is independent of the sequence of steps taken.) CROSSREFS Cf. A253057, A253058, A253952. Sequence in context: A094459 A108819 A158226 * A205273 A205266 A152834 Adjacent sequences:  A253950 A253951 A253952 * A253954 A253955 A253956 KEYWORD nonn,base AUTHOR Steve Butler, Jan 20 2015 STATUS approved

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Last modified April 8 01:45 EDT 2020. Contains 333312 sequences. (Running on oeis4.)