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A307603 Lexicographically earliest sequence with no duplicate term that produces only primes by the rounding technique explained in the Comments section. 1

%I

%S 1,5,2,3,4,6,7,10,52,53,11,12,58,59,13,16,60,61,17,18,66,67,19,22,70,

%T 71,23,28,72,73,29,30,78,79,31,36,82,83,37,40,88,89,41,42,96,97,43,46,

%U 502,503,47,100,508,509,101,102,520,521,103,106,522,523,107,108,540,541,109,112,546,547,113,126,556,557,127,130,562

%N Lexicographically earliest sequence with no duplicate term that produces only primes by the rounding technique explained in the Comments section.

%C See the assembly [a(n),a(n+1)] as a decimal number. Round this number to the closest integer. All rounded assemblies will produce a prime number.

%C "Rounding to the closest integer" is ambiguous for decimal numbers like (k.5) where k is an integer. Here we round such numbers to be rounded to k+1. The only occurrence of such a "rounding ambiguity" in the sequence happens with a(1) = 1 and a(2) = 5. Indeed, no more (k.5) "dilemmas" like that one will ever occur again as the integers 50, 500, 5000,... (that might produce together with the previous term k the decimal number k.50 or k.500 or k.5000...) cannot be part of the sequence; this is because 50, 500, 5000,... are not primes themselves (they end with 0) and neither are 51, 501, 5001,... (they are divisible by 3).

%H Jean-Marc Falcoz, <a href="/A307603/b307603.txt">Table of n, a(n) for n = 1..5001</a>

%e The sequence starts with 1,5,2,3,4,6,7,10,52,53,11,12,58,59,13,...

%e The assembly [a(1),a(2)] is 1.5 which rounded upwards produces 2;

%e The assembly [a(2),a(3)] is 5.2 which rounded to the closest integer produces 5;

%e The assembly [a(3),a(4)] is 2.3 which rounded to the closest integer produces 2;

%e The assembly [a(4),a(5)] is 3.4 which rounded to the closest integer produces 3;

%e The assembly [a(5),a(6)] is 4.6 which rounded to the closest integer produces 5;

%e etc.

%Y Cf. A173919 (Numbers that are prime or one less than a prime).

%K base,nonn

%O 1,2

%A _Eric Angelini_ and _Jean-Marc Falcoz_, Apr 18 2019

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Last modified March 30 22:55 EDT 2020. Contains 333132 sequences. (Running on oeis4.)