A072147 -id:A072147 - cp's OEIS frontend

A072137 Length of the preperiodic part of the 'Reverse and Subtract' trajectory of n.

0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 6, 4, 5, 3, 3, 5, 4, 6, 2, 1, 2, 1, 2, 6, 4

Offset: 0

Klaus Brockhaus, Jun 24 2002

'Reverse and Subtract' (cf. A070837, A070838) is defined by x -> |x - reverse(x)|, where reverse(x) is the digit reversal of x.
For every n the trajectory eventually becomes periodic, since 'Reverse and Subtract' does not increase the number of digits and so the set of available terms is finite. For small n the period length is 1, the periodic part consists of 0's, the last term of the preperiodic part is a palindrome.
The first n with period length 2 and a nontrivial periodic part is 1012 (cf. A072140).
This sequence is a weak analog of A033665, which uses 'Reverse and Add'.

			a(15) = 4 since 15 -> |15- 51| = 36 -> |36 - 63| = 27 -> |27 - 72| = 45 -> |45 - 54| = 9.

R. Zumkeller, Table of n, a(n) for n = 0..10000

Cf. A033665, A070837, A070838, A072138, A072139, A072140, A072141, A072146, A072147.

import Data.List(inits, find); import Data.Maybe(fromJust)
a072137 :: Int -> Int
a072137 = length . fst . spanCycle (abs . a056965) where
   spanCycle :: Eq a => (a -> a) -> a -> ([a],[a])
   spanCycle f x = fromJust $ find (not . null . snd) $
                              zipWith (span . (/=)) xs $ inits xs
                   where xs = iterate f x
-- Reinhard Zumkeller, Oct 24 2010

a[n_] := (k = 0; FixedPoint[ (k++; Abs[# - FromDigits[ Reverse[ IntegerDigits[#] ] ] ]) &, n]; k - 1); Table[ a[n], {n, 0, 104}] (* Jean-François Alcover, Dec 01 2011 *)

A072146 Numbers n with property that n sets a new record for the length of the preperiodic part of the 'Reverse and Subtract' trajectory of n.

Original entry on oeis.org

0, 1, 10, 13, 1011, 1013, 1028, 100103, 100107, 100184, 100368, 100484, 113008, 150703, 10000235, 10000434, 10001104, 10001285, 10001777, 10001788, 10002395, 10002494, 10109075, 1000002787, 1000011068, 1000011159, 1000012729, 1001050344

Offset: 1

Klaus Brockhaus, Jun 24 2002

A072147 gives the corresponding records. Since for small n the last term of the preperiodic part is a palindrome (cf. A072137), this sequence is a weak analog of A065198, which uses 'Reverse and Add'.

			The preperiodic part of the trajectory of 13 has length 6: 13 -> 18 -> 63 -> 27 -> 45 -> 9; for k < 13 the preperiodic part has a smaller length (at most 2).

Alexander Pesch, May 29 2007, Table of n, a(n) for n = 1..44

Cf. A072137, A072147, A065198.

a(19) = 10001777 inserted by Alexander Pesch, May 29 2007

Edited by N. J. A. Sloane, Dec 01 2007

cp's OEIS Frontend

A072137 Length of the preperiodic part of the 'Reverse and Subtract' trajectory of n.

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