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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified October 26 21:33 EDT 2021. Contains 348269 sequences. (Running on oeis4.)