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A270074 Select a pair of successive terms a and b; switch the two digits in contact with the comma between a and b; the new pair is now made of prime numbers. By decree no term is repeated in the sequence and all terms are nonprime numbers. This sequence is the lexicographically first with those properties. 1

%I #23 Nov 08 2016 20:48:26

%S 1,21,33,77,9,27,39,111,301,703,91,707,93,711,99,729,737,309,741,303,

%T 713,913,117,319,119,341,731,339,129,747,333,147,901,749,371,351,357,

%U 343,717,909,753,153,159,141,903,759,171,363,767,133,183,189,177,361,763,917,143,931,779

%N Select a pair of successive terms a and b; switch the two digits in contact with the comma between a and b; the new pair is now made of prime numbers. By decree no term is repeated in the sequence and all terms are nonprime numbers. This sequence is the lexicographically first with those properties.

%C Numbers that are composite for all last digits 1..9 cannot be part of the sequence, e.g., 201..209, 321..329. (Cf. A078492)

%H Lars Blomberg, <a href="/A270074/b270074.txt">Table of n, a(n) for n = 1..10000</a>

%e The first pair of nonprime numbers is 1,21. After we switch the digits of 1,21 we get 2,11 (which are indeed both prime numbers). The second pair 21,33 (both nonprime numbers) becomes 23,13 (prime numbers). The third pair 33,77 (both nonprime numbers) becomes 37,37 (prime numbers), etc.

%K nonn,base

%O 1,2

%A _Eric Angelini_ and _Lars Blomberg_, Mar 10 2016

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Last modified March 29 03:51 EDT 2024. Contains 371264 sequences. (Running on oeis4.)