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 A342078 Odd digits only come in successive pairs (separated or not by a comma). 3

%I

%S 1,10,2,3,30,4,5,50,6,7,70,8,9,90,11,20,21,12,22,23,31,13,32,24,25,51,

%T 14,26,27,71,15,52,28,29,91,16,33,40,41,17,72,42,43,34,44,45,53,35,54,

%U 46,47,73,36,48,49,92,55,60,61,18,62,63,37,74,64,65,56,66,67,75,57,76,68

%N Odd digits only come in successive pairs (separated or not by a comma).

%C The sequence starts with a(1) = 1 and is always extended with the smallest positive integer not yet present that does not lead to a contradiction.

%e a(1) = 1 forces the next digit to be a 1 (as odd digits must come in pairs); the smallest positive integer not yet present that starts with a 1 and does not lead to a contradiction is 10. Thus, a(2) = 10;

%e a(3) = 2 as the smallest positive integer not yet present that does not lead to a contradiction is 2;

%e a(4) = 3 as the smallest positive integer not yet present that does not lead to a contradiction is 3;

%e a(5) = 30 as a(5) must start with a 3 (the odd digits must come in pairs) and 30 is the smallest positive integer not yet present that does not lead to a contradiction; etc.

%o (Python) # see A342076 for aupton, pairsup

%o mustpair = {1, 3, 5, 7, 9}

%o print(aupton(67)) # _Michael S. Branicky_, Feb 28 2021

%Y Cf. A342076, A342077 and A342079 (variations on the same idea).

%K base,nonn

%O 1,2

%A _Eric Angelini_, Feb 28 2021

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Last modified June 29 05:16 EDT 2022. Contains 354910 sequences. (Running on oeis4.)