

A342077


Prime digits only come in successive pairs (separated or not by a comma).


3



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, 19, 22, 33, 40, 41, 42, 23, 32, 24, 43, 34, 44, 45, 52, 25, 53, 35, 54, 46, 47, 72, 26, 48, 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
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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.


LINKS

Table of n, a(n) for n=1..77.


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

Cf. A342076, A342078 and A342079 (variations on the same idea)
Sequence in context: A210416 A220943 A082259 * A077339 A077341 A344545
Adjacent sequences: A342074 A342075 A342076 * A342078 A342079 A342080


KEYWORD

base,nonn


AUTHOR

Eric Angelini, Feb 28 2021


STATUS

approved



