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A303845
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A fractal-like sequence: erasing all pairs of consecutive terms that produce a prime by concatenation leaves the sequence unchanged.
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15
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1, 2, 3, 2, 4, 7, 5, 9, 3, 2, 4, 6, 13, 8, 11, 7, 5, 10, 19, 12, 17, 14, 23, 15, 31, 9, 3, 2, 4, 6, 16, 21, 18, 47, 13, 8, 20, 27, 22, 37, 11, 7, 5, 10, 24, 41, 19, 12, 25, 39, 26, 33, 17, 14, 28, 43, 23, 15, 29, 53, 31, 9, 3, 2, 4, 6, 16, 30, 49, 21, 18, 32, 51, 34, 57, 47, 13, 8, 20, 35, 59, 36, 71, 38, 63, 27, 22, 40, 73
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OFFSET
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1,2
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COMMENTS
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The sequence is fractal-like as it embeds an infinite number of copies of itself.
The sequence was built according to these rules (see, in the Example section, the parenthesization technique):
1) no overlapping pairs of parentheses;
2) always start the content inside a pair of parentheses with the smallest integer P > 1 not yet present inside another pair of parentheses;
3) always end the content inside a pair of parentheses with the smallest integer R > 1 not yet present inside another pair of parentheses such that the concatenation PR is prime;
4) after a(1) = 1, a(2) = 2, a(3) = 3, always try to extend the sequence with a duplicate > 1 of the oldest term of the sequence not yet duplicated; if this leads to a contradiction, open a new pair of parentheses.
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LINKS
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EXAMPLE
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Parentheses are added around each pair of terms whose concatenation produces a prime:
1,(2,3),2,(4,7),(5,9),3,2,4,(6,13),(8,11),7,5,(10,19),(12,17),(14,23),(15,31),9,...
Erasing all the parenthesized contents yields
1,(...),2,(...),(...),3,2,4,(....),(....),7,5,(.....),(.....),(.....),(.....),9,...
We see that the remaining terms rebuild the starting sequence.
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CROSSREFS
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Cf. A000040 (the prime numbers), A303950 (remove parentheses with Fibonacci sum).
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KEYWORD
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AUTHOR
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STATUS
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approved
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