A072139 - OEIS

%I #4 Jul 11 2015 00:36:41

%S -1,1,2,3,4,5,6,7,8,9,9,11,9,9,9,9,9,9,9,9,9,9,22,9,9,9,9,9,9,9,9,9,9,

%T 33,9,9,9,9,9,9,9,9,9,9,44,9,9,9,9,9,9,9,9,9,9,55,9,9,9,9,9,9,9,9,9,9,

%U 66,9,9,9,9,9,9,9,9,9,9,77,9,9,9,9,9,9,9,9,9,9,88,9,9,9,9,9,9,9,9,9,9,99,99,101,99

%N Last term of the preperiodic part of the 'Reverse and Subtract' trajectory of n, or -1 if the trajectory is completely periodic.

%C 'Reverse and Subtract' (cf. A072137) is defined by x -> |x - reverse(x)|. For small n the positive terms are the first palindrome in the trajectory of n, so this sequence is a weak analog of A033865, which uses 'Reverse and Add'. a(1012) = 8712 is the first non-palindrome (cf. A072140). For k in A072140, A072141 or A072142 we have a(k) = -1.

%e a(0) = -1, since 0 -> |0 - 0| = 0, the preperiodic part is empty; a(12) = 9, since 12 -> |12 - 21| = 9.

%Y Cf. A033865, A072137, A072140, A072141, A072142.

%K base,easy,sign

%O 0,3

%A _Klaus Brockhaus_, Jun 24 2002