This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A075607 #21 Feb 20 2024 07:00:57
%S A075607 1,3,7,9,11,17,21,13,19,31,37,27,29,39,23,33,43,49,51,47,59,53,81,61,
%T A075607 63,67,79,93,41,57,83,69,71,77,89,99,73,121,97,87,103,91,127,123,113,
%U A075607 111,109,133,117,101,107,119,129,169,151,141,131,143,137,147,139
%N A075607 a(1) = 1, a(n) = smallest number not occurring earlier such that the concatenation a(n-1) and a(n) is a prime.
%C A075607 Almost certainly a permutation of A045572. - _David W. Wilson_, Jan 15 2005
%H A075607 Paolo P. Lava, <a href="/A075607/b075607.txt">Table of n, a(n) for n = 1..10000</a>
%t A075607 a = {{1}}; Do[k = 2; While[Nand[! MemberQ[a, #], PrimeQ@ FromDigits@ Join[a[[n - 1]], #]] &@ Set[d, IntegerDigits@ k], k++]; AppendTo[a, d], {n, 2, 61}]; FromDigits /@ a (* _Michael De Vlieger_, May 08 2017 *)
%o A075607 (PARI) A075607(n,show=0,a=1,u=[])={for(n=2,n,show&&print1(a","); u=setunion(u,[a]); while(#u>1&&u[2]==u[1]+2,u=u[2..-1]); forstep(k=u[1]+2, 9e9, 2, setsearch(u,k)&&next; isprime(eval(Str(a,k))) && (a=k) && break)); a} \\ Use 2nd, 3rd or 4th optional arg to print intermediate terms, to use another starting value or to exclude some terms. - _M. F. Hasler_, Nov 25 2015
%Y A075607 Cf. A075608, A075609, A075610.
%K A075607 base,nonn
%O A075607 1,2
%A A075607 _Amarnath Murthy_, Sep 28 2002
%E A075607 More terms from _Michel ten Voorde_ Jun 23 2003