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%I #11 Feb 28 2021 10:10:30
%S 1,2,20,3,30,4,5,50,6,7,70,8,9,10,11,12,21,13,31,14,15,51,16,17,71,18,
%T 19,22,33,40,41,42,23,32,24,43,34,44,45,52,25,53,35,54,46,47,72,26,48,
%U 49,55,60,61,62,27,73,36,63,37,74,64,65,56,66,67,75,57,76,68,69,77,80,81,82,28,83,38
%N Prime 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(2) = 2 forces the next digit to be a 2 (as prime digits must come in pairs); the smallest positive integer not yet present that starts with a 2 is 20. Thus, a(3) = 20;
%e a(4) = 3 as 3 is the smallest positive integer not yet present that does not lead to a contradiction;
%e a(5) = 30 as 30 is the smallest positive integer not yet present that does not lead to a contradiction and starts with a 3; etc.
%o (Python) # see A342076 for aupton, pairsup
%o mustpair = {2, 3, 5, 7}
%o print(aupton(67)) # _Michael S. Branicky_, Feb 28 2021
%Y Cf. A342076, A342078 and A342079 (variations on the same idea)
%K base,nonn
%O 1,2
%A _Eric Angelini_, Feb 28 2021
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