A275680 - OEIS

%I #13 Mar 23 2017 14:33:45

%S 1,9,70,10,12,72,74,30,75,14,90,76,15,16,33,18,100,91,92,93,77,34,78,

%T 700,102,104,94,105,106,35,36,702,703,95,38,39,704,300,108,705,706,96,

%U 707,98,301,99,110,302,303,710,304,111,712,112,306,115,116,308,310,900,117,118,119,312,120,121,316,318,122,319,123,124,901,125,902,904

%N Nonprime integers that become prime when concatenated with the next digit.

%C The sequence is started with a(1) = 1 and always extended with the smallest integer not yet present that does not lead to a contradiction.

%H Jean-Marc Falcoz, <a href="/A275680/b275680.txt">Table of n, a(n) for n = 1..10001</a>

%e 1 is not prime but 19 is [1,9].

%e 9 is not prime but 97 is [9,7(0)].

%e 70 is not prime but 701 is [70,1(0)].

%e 10 is not prime but 101 is [10,1(2)].

%e 12 is not prime but 127 is [12,7(2)].

%e 72 is not prime but 727 is [72,7(4)].

%e 74 is not prime but 743 is [74,3(0)].

%e 30 is not prime but 307 is [30,7(5)].

%e 75 is not prime but 751 is [75,1(4)].

%e 14 is not prime but 149 is [14,9(0)].

%e ...

%K nonn,base

%O 1,2

%A _Eric Angelini_ and _Jean-Marc Falcoz_, Aug 05 2016