OFFSET
1,2
COMMENTS
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.
EXAMPLE
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;
a(4) = 3 as 3 is the smallest positive integer not yet present that does not lead to a contradiction;
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.
PROG
(Python) # see A342076 for aupton, pairsup
mustpair = {2, 3, 5, 7}
print(aupton(67)) # Michael S. Branicky, Feb 28 2021
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Eric Angelini, Feb 28 2021
STATUS
approved